if you invest in altcoin popular or among the top 10 in CMC I don't think you need to worry about them, I'm sure that among them it won't make you lose big. but if you invest in altcoins that don't have price developments, it's very risky. but for the convenience of your money, I think you need to invest with bitcoin.
what you say is true if all depends on what altcoin we have because there are many pretty good altcoins that are even better held in the near future than Bitcoin. so if we feel our coins are good enough, we don't need to exchange them for bitcoin or any coins.