No One knows that there always could be deeper and also higher , if you are long enough in crypto and have seen 2017, everyone would think altcoins would go higher and there was a bubble that burst in 2018 and 2019, they thought there is no deeper but there was ,always was, some altcoins dead officially and people lost money due to holding shitcoins because they thought they would go up but for most of them never happened, the same thing happened to me too when I held a bag of shitcoins and most of them are dead, but only the best ones could survive , So take out the profit when you can and cut the greed.
When it comes to altcoins, we must be very actively trading them or look into them as they can do exit in the market when they wanted. There are already known projects before that we all thought are good ones but just left their investors hanging. Alts were good for trading, others were able to earn by trading them often that holding them. My fault before is I choose to hold them thinking that just like bitcoin they can arise in price when crypto adoption happens but it's only applicable in some other alt coins, others were just meant to be Delisted and gone.