I joined crypto in 2017 and until now I still find it difficult to find or choose which project I should hold for a long time and it is true that the safest point is to keep your assets in the top 20 coinmarketcap, at least the project already has a very clear roadmap and team.
Choosing the top coins are best for newbies because these coins are already well-established and already here for years. Its unlikely to be delisted in exchanges so even you hold it for long period you dont have to worry. On the other side new coins has risk of it being scam even the project seems good. Scammers are smart to make it look legit so think before investing.