None of those are major cryptocurrencies, and the blockchains of many of those which are now effectively worthless continue to exist in a handful of nodes around the world, showing the resilience of this enterprise structure.
>>A digital cash platform will shut down when it's not profitable and nobody wants to use it anymore, the exact same thing is true of any cryptocurrency.
No, a blockchain is inherently far more widely distributed in its infrastructure providers and datastores, and is therefore far less likely to disappear as a trusted database for shared operations like a token ledger.
>>that's it's actually a good thing that BTC and ETH are getting "too big to fail"
In this case they are too big to fail on their own, not too big for society to allow them to fail, which is an entirely different concept, and only superficially similar. It is a good thing for the world at large when a shared ledger becomes increasingly robust to failure due to its own structure becoming more resilient. This makes for a more reliable platform for financial and social interaction.