I feared new altcoins will be left out so i start selling some coins for bitcoin because it can be a big blow if altcoin fails to surge like in 2017 after bitcoin reach a new ATH, what do you think? And what would you have done in my position
It is true that most altcoins would be left out to die but it really depends on the type of altcoin. There are a great number of altcoins today and holding the one with no prospect will only lead to loss, holding the pump and dump altcoins is also a bad decision. Now, about selling your altcoins to BTC is a good move as holding BTC is always the best to hold, coupled with few altcoins so as to balance everything. So the idea is, hold good altcoins like ETH and hold BTC in a good amount.