How and why would the end of bitcoin (and any other cryptocurrency) occur? Value decrease? Legality issues? Different technology/ideas to supersede it?
Currently since bitcoin is growing, I don't see any reason why bitcoin would die in any perspective. I'm optimistic in what I'm investing thus I don't consider such things that could affect me negatively and something I know that will not happen. If people have an optimistic mind, none of negative will happen.Said the dinosaur watching the asteroid.
I think it would only really die if the internet did not exist anymore, that's all.
Actually no.
As long as there is a single miner just like satoshi bitcoin would still be able to keep being "alive" even in a closed network.
Banning bitcoin and other cryptos by the government is not enough to make them die. Because as long as we are still here, the market is there and people will still use it, bitcoin will go undeeground first, it isn't going to die 
And even if ISP's will block connections, I feel as though someone would make a work around, we are dealing with open sourced code here.
And even if ISP's will block connections, I feel as though someone would make a work around, we are dealing with open sourced code here.Don't be so sure.
Bitcoin is based on the people interest and faith in it.
Once there is a strong assault against it followed by arrests and criminal charges people will start losing interest in it.
Just imagine that all the exchanges would have their accounts frozen, who would still be trading?
Betting websites would have their domains and nameservers suspended. Atms will be closed.
Stores will stop accepting btc.
Returning to the darkmarkets only?
Not an option.
But it won't happen.... I hope