I don't think it's a problem if you only invest in 1 coin, as long as the investment is in bitcoin. but sometimes many people who want more profit so choose altcoins to invest. but for a smart investor, of course he will divide his capital into several altcoins so that by diversifying it, will make the risk lower
it all depends on the selected asset. not just bitcoin I guess. to maximize profits if you have not too large capital, there are many other potential altcoins such as DOT, ETH, and BNB that can be an option. and I'm sure those assets won't end badly. but it also depends on our ability to manage our psychology to face the market.