A lot of the "real money" startups generate is from selling services to other startups and tech companies - particularly in the YC ecosystem. It's possible that revenue will slow in a crash and start a nasty cycle.
I suspect something similar will happen this time around -- there will be a slowdown in the real economy for some unpredictable reason, and the imaginary, startup-servicing-startups economy will implode.
More broadly, an even larger number of startups are also indirectly dependent on the current landscape, as they operate in regional markets where consumption is being driven in large part by the influx of capital into tech.
There's nothing wrong with building a business that takes advantage of these types of situations, but my sense is that a lot of the entrepreneurs running these types of companies aren't consciously aware that they're the beneficiaries of an environment that will eventually change. As such, they're not at all focused on trying to diversify their revenue base and to be frank, it many cases there really isn't an opportunity for them to do so anyway.