Decentralized cryptos will last for as long as there are adopters.

The reason is that almost all cryptos are decentralized (except for say XRP and a few others), and we know that if there is even just a few nodes on the network, it can survive. The only thing that can kill crypto is if public interest drops to 0, which I think is unlikely in a long amount of time.

But whether the value holds up is a different thing altogether.