When Nicola Sturgeon announced she was quitting the stage, the most extravagant bouquets of compliments were thrown not by her friends, but her adversaries. “A gamechanger,” whooped excitable Labour people, instantly and rather prematurely imagining the parliamentary seats they could bag with the SNP deprived of her star power. “The union is safe,” gasped euphoric Tories, immediately and rashly thinking that they could stop worrying about Scotland separating itself from the UK. It is not hard to see why opponents were so gladdened by her departure. She led her party at three general elections and two for the Scottish parliament. Five contested, five won. Each by crushing margins over her competitors. Other politicians would exchange one of their lungs for an electoral record like that. She had staying power. There have been five tenants of Number 10 during her eight years as first minister of Scotland. As is usually the case with longstanding leaders, Ms Sturgeon shone brightly at her zenith, but was flickering towards the end. It wasn’t just the personal burnout that she talked about in her resignation statement at Bute House. There’s also the political meltdown she was less keen to discuss. Her government has been engulfed by a torrid sequence of troubles, from a costly fiasco over ferry contracts which has become emblematic of SNP misgovernance to swelling criticism of the party’s failures in delivering public services. Then there’s the non-trivial matter of a police investigation into the financing of the party, a murky affair in which her husband is embroiled. She made herself unpopular with her position on gender identity and handling of the incendiary controversy triggered by the revelation that a double rapist had been placed in an all-female Scottish prison. When Rishi Sunak wielded Westminster’s veto over the gender recognition legislation passed by Holyrood, it was a highly rare case of a Tory prime minister being more aligned with opinion north of the border than Scotland’s leader. I largely believed her when she said that she wasn’t buckling under these “short-term pressures”, but only because I think her core reason for resigning now is that she can’t glimpse any route to a winnable referendum on independence in the foreseeable future. There are several reasons why she will be remembered, one of them for being Scotland’s first female first minister, but she did not make the history that she most wanted to. A woman who has been campaigning for independence since she was a teenager surely would not be walking away if she thought there were a realistic chance of becoming the first prime minister of a sovereign Scotland. The SNP is losing its most formidable advocate and managing the tensions between the party’s fundamentalists and its incrementalists will be a huge test for whoever comes next. There is no heir apparent, no shoo-in for the succession, as she was when she was crowned leader without a contest in 2014. Some of her potential successors have been around too long to be a refresh; others haven’t been around long enough to be properly tested. None have her array of communication skills, guile, resilience, experience and authority. That will make it tougher for the next leader to deal with the fact that the SNP’s hopes of forcing another referendum have reached a constitutional dead end. The supreme court has ruled that they can’t hold one without the agreement of the Westminster government. The Tories won’t accede to that and Sir Keir Starmer says they won’t get a second plebiscite from a Labour government. Ms Sturgeon’s fallback plan, to attempt to turn the next general election into a proxy referendum, is highly contentious within her own party, disliked by voters and may well be ditched. Rival parties add that the SNP’s record in power is finally catching up with them. The most potent critique is that they have been first-class campaigners, but fifth-rate governors. That is meant as devastating commentary on the SNP, but it is an even more severe indictment of the unionist parties. If the record has been as terrible as it is depicted by Labour and the Tories, they have to ask themselves why they have failed to deny the SNP 16 uninterrupted years of power at Holyrood. As the polls indicate, support for independence ebbs and flows, but never that decisively one way or the other. Scotland is essentially a 50-50 nation. This deadlock makes everyone miserable. The unionist parties are frustrated that they can’t dislodge the SNP. The dominant party is maddened that it can’t achieve its paramount ambition. Despite Brexit and more than a decade of Tory governments, neither of which were wanted by most Scots, the SNP has never achieved a sustained majority for independence at a level that would force Westminster to concede another referendum. Despite the increasingly poor record of the SNP as a government and the gaping holes in its prospectus for a breakaway, the unionist parties have failed to make either themselves or being part of the UK appealing to enough Scots to put the independence question to bed. Some of the pro-union commentary from south of the border has been overly intoxicated with the thought that Ms Sturgeon’s exit secures the longevity of the United Kingdom. Support for independence is not rooted in just one personality. It is much more structural than that. The independence movement is not going to evaporate overnight just because she is going. Time is against the union. There is a chunky majority for independence among Scots who are younger than 50. There’s also evidence that Scots don’t change pro-independence views as they get older. If demography is destiny, and if something big does not change, Scotland is on a trajectory towards separation at some point. That ought to concentrate minds at the Scottish Labour conference this weekend. The gathering in Edinburgh is buoyant and naturally preoccupied with how Sir Keir Starmer’s party can capitalise on the resignation of Ms Sturgeon and the ensuing turmoil in her party. If Labour can’t exploit that, it will be a golden opportunity squandered. Labour has just one MP representing a Scottish seat and Sir Keir’s path to Number 10 will be easier if his party can improve on that dismal tally. I don’t meet anyone who thinks Labour will ever restore the iron grip it once had on Scotland, but there is building hope in its ranks that their prospects are improving. One indicator of that is a return to the frontline of politics by Douglas Alexander, who was in the cabinet during the New Labour era. Mr Alexander played a key role in the successful “better together” campaign to keep Scotland within the UK in 2014, only to be evicted from the Commons when all but one of the Labour MPs north of the border were obliterated at the 2015 election. He has just been selected as Labour’s candidate in East Lothian, the party’s top target seat in Scotland. He wouldn’t be re-entering the fray without a lot of conviction that he will take the seat, Labour will form the next government, and he might find himself at the cabinet table again. Party strategists plan to try to mobilise pro-union voters behind Labour candidates. But to make substantial gains north of the border Labour will also need to lure over voters who have previously backed the SNP. Hardcore supporters of independence will be unbiddable. Labour’s efforts will be focused on those voters who are only moderately in favour of independence and animated by non-constitutional issues such as the state of public services, crime and the cost of living. These “soft Yes” voters will be decisive at the next election. If lots of them can be pulled over, Labour has the possibility of winning basketfuls of seats north of the border. If “soft Yes” voters find Labour unpersuasive, its gains are more likely to be measured in handfuls. That could be the difference between a majority Labour government and a minority one. Sir Keir, who addresses the Edinburgh conference today, has three essential tasks. To convince Scots that voting Labour is the only guaranteed way to end Tory rule at Westminster. To give them reasons why their lives will be better for having a Labour government. And to project his party as a faster, better way of achieving change than pursuing independence. If he makes it to Downing Street, he will then face the longer-term challenge of saving the union. That can only be done by persuading more Scots than currently feel this way that the UK is a hospitable home in which they can thrive. Absent that, the independence question is not going away. It will still be live long after Ms Sturgeon has published her memoirs. Andrew Rawnsley is Chief Political Commentator of the Observer