There's actually two ways for Bitcoin to get replaced by another crypto-currency. The first and a far-from-reality way is an altcoin would rise. That altcoin is not a simple altcoin, the devs of that altcoin has implemented out-of-the-box ideas that people of the Internet has never thought of. Sounds unrealistic, right? But it might be possible in the future, we just can't see it right now.
The second way is that Bitcoin gets taken down by something. Maybe hackers would DDOS the shit out of Blockchain. Idk what it would be but it's something that will make Bitcoin transactions really really slow, making people go to the second best crypto-currency at that point.
Your second way of moving Bitcoin out of existence seems to be even more far-fetched
At least, in respect to outside actions against Bitcoin. Really, if someone is going to beat the shit of Bitcoin, you immediately have to face two questions, at the very minimum. First, why it hasn't been done already, and, second, if it is possible why another coin will be invulnerable to this attack? As to me, Bitcoin, as of now, can be destroyed only from inside. It has existed long enough to be considered as immune or strongly resistant to anything we have seen so far (DDOS, spam attacks, and whatnot) as well as free of potentially lethal bugs, for absolutely the same reason