It's kinda inevitable that there will be one or several big crashes coming up. However, it's far from trivial to say when. Perhaps when the market reaches 200b, 1t or 10t - who knows?
The technology has a lot of use cases, meaning even a crash or two won't eliminate the market. But it will kill a lot of the altcoins I'd wager.
The dot-com bubble didn't result in the internet becoming worthless, just a bunch of the companies trying to profit from it (the bottom lot)
Yes, there is a lot of rubbish out there, but a lot of good stuff too, that meets real needs.
Bitcoin will I think supplant gold as the principal store of value. Something like ETH is needed too; blockchain smart contracts will be extremely important. Same goes for XRP, eliminating inefficiencies in international banking. And similar statements can be made down through the list... but as marketcap falls, you hit more and more coins that are either copies of other coins, have no use, or are outright worthless.