Why shouldn't sell altcoins and buy Bitcoin? In my opinion, Bitcoin is the best option for long-term holding. Altcoins may have good short-term growth potential, but they are all dependent on Bitcoin. So why not just buy Bitcoin and hold it?
We should always leave 80 % to 60% of our crypto portfolio in Bitcoin because if any of the altcoins we are holding eventually crashes or the price keep going down, allowing us to make losses, we could be surprised that those of us that have higher percentage of Bitcoin in our portfolio will not be bothered. Investors need to be aware that the crypto market is very risky and luck does not happen here always.
You could buy a coin and the next day, you could be surprised that the price has crashed and there is no way you could get money to help yourself if that's someone that like putting all theirs eggs in one basket.