How big can the largest tech companies get? How completely can they come to dominate the economy? The "big five" -- Apple, Alphabet, Microsoft, Facebook and Amazon -- now have a combined valuation of over $3.3 trillion, and make up more than 40 percent of the value of the Nasdaq 100 index. As the digital economy continues to grow faster than the old economy, its hard to see what can stop these juggernauts. Unless reality intrudes. After all, what exactly is their business? Who are their customers? What role do they play in the economy? Each answer points toward some limit on the size, scale and profitability of these giants. These companies are big for a reason: Nearly every aspect of the digital economy touches them in some way or another. We know that Facebook and Google represent a digital advertising duopoly. We know that Amazon is gobbling up more and more of e-commerce. Amazon, Google and Microsoft are leaders in providing cloud services. Apple sells high-margin smartphones and other computing devices. Put it all together, and youre talking about hundreds of billions of dollars of annual revenue and tens of billions of dollars in profits. Whats forgotten as these companies seemingly gobble up the rest of the economy is they remain dependent upon customers who get value from their services. Companies advertise on Facebook and Google only if theyve determined its more profitable than not doing so. Cloud revenue requires the existence of profitable businesses that need business software and services. Third-party vendors choose to sell on Amazon because its profitable for them to do so. In other words, for the most part, the big five tech companies exist at their current size and scale only because they serve a larger underlying economy of profitable companies. But the disruptive nature of the tech companies raises questions about how much they can grow. Because, in a sense, at some point theyll only be able to grow by putting some of their customers out of business either directly or indirectly. Consider a couple examples. Blue Apron, a meal delivery company that went public this year, has been a prolific advertiser online. If Amazon came out with a competing service that put them out of business, Facebook and Google would lose out on some advertising revenue, and Microsoft and Google (and Amazon) might lose some cloud revenue. Another company, Fossil Group, has struggled mightily over the past several quarters as consumers have bought fewer watches, perhaps in part because of the Apple Watch. If the Apple Watch disrupts Fossil, Facebook and Google would lose ad sales from Fossil, and Amazon would lose Fossil watch sales as well. The retail vision put out by some tech optimists would be devastating to overall advertising revenue. Imagine it in the extreme: If Amazon put all physical retail stores out of business, and private-label goods replaced all branded goods, youd kill the source of a large swath of advertising demand. In a sense, Amazon could partially disrupt Facebook and Google without ever competing directly with them. These tech platforms and the companies they serve exist in an ecosystem, where there must be some sort of balance. Profitable companies can allocate only so much of their revenue to advertising, cloud services, information technology and the like. If their profits go away or are disrupted by tech companies, their ad and tech spending will go away. A few highly successful predators could decimate their ecosystem and wind up hungry. Markets and the overall economy got in trouble by making a similar mistake about another sector -- finance -- a decade ago. Finance, like these tech companies, exists as a layer on top of an underlying economy. Markets became irrational about how profitable the financial sector could become relative to the underlying economy, and in response to these market pressures, finance came up with increasingly elaborate schemes to make money that werent sustainable. We may not be quite there with tech yet, but as stock valuations climb higher and higher, tech will be feeling the same pressures that Wall Street did a decade ago. Expect a similar collapse of the ecosystem.Bloomberg