How and why would the end of bitcoin (and any other cryptocurrency) occur? Value decrease? Legality issues? Different technology/ideas to supersede it?
Very easy, only need the law prohibits people from using Bitcoin, ban all transactions on the internet, ban all mining Bitcoin ... if that will happen, Bitcoin only have one way is die 
. But I not hope that is true, because it is the end of the world for me 
The law could not prohibit, i think thats ist not possible btc die because legality
I did not get you there, are you saying that the governments will not prohibit it and it is impossible that BTC will die because it is legal?
If that is what you are trying to say, my friend you are wrong, the government are not the ones handling BTC, in fact no one is thus decentralized and there are already countries that have banned BTC because of legalities.
This will not happen if the bitcoin will die in a law because it stated that occur only on the legality of the State only certain countries or because of a decline in the value of bitcoin but now more and more are using bitcoin. If you think bitcoin won't die all the time in fact will be the digital currency for the future in the future because there are already a number of countries that ratified the bitcoin in General