Bitcoin has dominated my investment portfolio for years, there aren't many altcoin I have considered since then other than a few of the top altcoin. Investment portfolio tend to be good when there aren't a lot of shitcoin, even if bitcoin is the single most dominant but altcoin are also profitable. So there's nothing wrong with altcoin, but they shouldn't dominate your portfolio in the long term. After all, mistakes like that were hard to predict, but avoidance would be better than cure.
I think that it is very important to diversify your portfolio, even if you have a specific project that you are convinced, that this is the best project of them all and that it will give you the best profit.
Many people do it, but not all of them are successful because there are many things that can influence them in choosing and determining the future of their portfolio. As much as possible avoid the hype on certain altcoin, especially if you want to make it a long-term investment.