It is unlikely that BTC will collapse just like that, because too much money has been invested, and this means that there is huge support from ordinary people, and not so long ago also from large investors. However, if we imagine the situation when something like that would happen, then altcoins would definitely experience a heavy blow, because most of them have no purpose and sense, and in an unprecedented panic, they would probably disappear in a bloodbath.
The only question is whether someone would appear who would manage to position one of the more famous alts as the successor to BTC in such a chaotic situation. However, BTC is the only truly decentralized coin, and it is impossible to find a replacement - such things are actually in the domain of science fiction.