Obviously, something will take it's place.
The thing is, btc won't really ever die.
Sure the value can go lower, but blockchain technology isn't something you just kill off.
It is an open ledger distributed across the world.
As long as people mine it, trade it, support it, it won't die.
Just look at doge, that thing is a literal jokecoin that has lasted all these years and is in the top 100.
The more realistic scenario, is not that there will be eth or xrp that will replace btc, but two or three companies with similar market impact.
As crypto grows, more and more trading pairs and ways to liquidate will become avaiable meaning we won't have to rely on just one coin.
This always happens to markets that mature over time.
I think this scenario could theoretically come true in 30-50 years, but in practice it will never happen. Bitcoin can't become a stale product because it is constantly evolving. The community behind it is so smart that they will never accept a situation when other coins are better, they will keep Bitcoin always up-to-date, so to speak. Besides, the "advantages" of some current altcoins are questionable, because it's unknown how would they behave in case they were tested to the degree Bitcoin was.
