Altcouns has it's own season, the time is coming when goo altcoins will move along with coin, if you want to sell your altcoins for btc you should have done that when bitcoin was still low let's say when it was 3k, this is not the right time to sell alt for btc.
The season of altcoins will come to us soon, because the season of bitcoin has come nearly to its end. It is so ridiculous to ask "Sell All Altcoins for Bitcoin", like the written OP. In contrast, now it is time to aks an opposite question: Is it time to sell all bitcoin for altcoins (good altcoins)? I think the latter question make more sense in current phase of crypto market. How long has bitcoin started its bull run? More than 6 months, that is long enough to hit its turning point, from which bitcoin domination will fall and altcoins will see their own season.