Of course no company can
increase their net profit YoY
sustainably, unless you mean increasing it at the rate of overall economic growth, because otherwise after a few decades the economy consists of only that company and its growth must then fall to the rate of overall economic growth.
So let's consider the more reasonable interpretation of "viable business models" as whether Uber or AirBnB can be sustainably profitable at a level that justifies their current market cap (and thus stock price.) Uber Cab is US$73B, AirBnB is US$100B, and justifying those at a P/E of 20 would require respectively US$3.5B and US$5.0B per year of profits, which are US$0.50 and US$0.75 per person per year, at the current population.
They're both effectively marketplaces making a living by skimming a commission off the earnings of actual service providers, who are providing respectively transportation and lodging. Such commissions are usually in the 0.5%-50% range depending on the market power of the marketplace and the kinds of risks involved. (Consulting agencies commonly suck off 50%, for example, in part because they don't get paid at all if the client is sufficiently unhappy.) Suppose they're 0.5%. Then to justify Uber's market cap, the annual transportation budget per person needs to be at least US$100, and the annual lodging budget per person needs to be at least US$150.
These seem like very plausible numbers to me, even a bit low. For example, right now Uber's commission is 20%, 40 times higher than the 0.5% I'm using above, and pre-covid the US hotel/motel industry was about US$220 billion for a country of only 330 million people, which is US$666 per person, a beastly number that is four times higher than the US$150 I computed above. So I think the market is pricing in a fairly large risk that Uber or AirBnB will just blow up and be worth $0 in a few years.
But I guess you see it very differently. What's your analysis? What am I missing?