I agree with you, when many people hear cryptocurrency, they immediately think Bitcoin. But the more they get to know more about crypto then they realize that Bitcoin is not the only crypto.
Bitcoin is the most popular and has the largest Market Cap value, when it moves all other coins moves, apart from some certain time but mostly, it carries the market along. So if Bitcoin is growing all other coins will grow with it, like you have said Bitcoin holders, still hold ETH and other Altcoins too.