How and why would the end of bitcoin (and any other cryptocurrency) occur? Value decrease? Legality issues? Different technology/ideas to supersede it?
Bitcoin dies? That can happen. When there is another kind of bitcoin that is better in value, less expensive, the amount of bitcoin users will decrease, which is of course because people are always looking for profit.
The price is the value of cryptocurrencies. But let's say you meant "value" as in its worth as something. I really doubt that bitcoin users would just move to that because it's better. You'd see tons of coins out there that claim to be better than bitcoins because they have slight faster confirmation times or that they have good developers. Those are not enough for people to untrust bitcoins and then migrate. Bitcoin has to die on its own.
We're all waiting for the lightning network technology. I even doubt that a coin with lightning network will overthrow bitcoin because bitcoin can just adopt to that. And, the whales don't mind the transaction fee at all. They can be the highest priority if they want since they can afford high fees.