Its impossible to have one or two coins in crypto space because one or two coins can't solve all the problems we have in this world in every aspect of technology or others, if you want to keep a coin for a long time make sure the coin or token solves one or two problems in crypto space or the world, this type of coins don't die easily because they have real use cases, its simple as that
I think there are different use cases for coins and that's why this belief has a point. If bitcoin can do all other coin or tokens worth then no need these what we called altcoins. Also different users prefer different kind of approach which some project totally unique. I like eth network so much more than bitcoin though bitcoin is a good asset which is likely to be the most useful in the current time.