what makes you think they will collapse? I think with the community and the strong popularity they have, it will be very difficult for them to really collapse. rising and falling prices are natural in crypto. price decline does not mean collapse.
btw in my opinion for now there are no altcoins that can replace bitcoin
to replace the bitcoin also did not easy. bitcoin too high achievements, most people's mindsets are already outrageously high reserved bitcoin. to replace the bitcoin for me would be a difficult thing to altcoin. Although they will be developed in line but not to beat.