How and why would the end of bitcoin (and any other cryptocurrency) occur?
In Bitcoin, the supply decreases in time and will reach its final number somewhere in around 2140
I've read it on blockgeeks.
Value decrease?
It's value depends on the supply and demand. If more people would invest in bitcoin, it's value will increase. But when a lot of people are withdrawing or converting it into cash it's value will drop.
Legality issues?
I'm not sure about it's legality. But you don't need anybody's approval to use cryptocurrencies. Nobody can prevent us from using it unless it is prohibited by your government.
Different technology/ideas to supersede it?
If someone would invent something more promising than bitcoin.