By CONOR SEN / Pic By BLOOMBERG

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 US$3.3 trillion (RM13.78 trillion), and make up more than 40% of the value of the Nasdaq 100 index.

As the digital economy continues to grow faster than the old economy, it’s 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 you’re talking about hundreds of billions of dollars of annual revenue and tens of billions of dollars in profits.

What’s forgotten as these compaConsider 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, you’d 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 eco-system, 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 under-lying economy.

Markets became irrational about how profitable the financial sector could become relative to the under- lying economy, and in response to these market pressures, finance came up with increasingly elaborate schemes to make money that weren’t 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 col- lapse of the ecosystem. — Bloomberg

• This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.