Look at it this way: People are selling altcoins for BTC because they're FOMOing the current pump. They are leaving altcoins at ATLow and entering BTC near ATHigh.
What do you think it's gonna happen? The answer is obvious, people entering near all time high on BTC will get crushed and then they will come back to altcoins after they are already on the move.
Don't panic sell at the bottom, the time to sell altcoins is long gone, if you're on the loss with altcoins, hold on to it because trying to time the market is super hard (Not financial advice)
While you make some interesting points I do not agree with them, it is true that people are selling altcoins in order to buy bitcoin because of the fear of missing out but if the price of bitcoin begins to crash they are not going back to altcoins because altcoins are going to be crashing even faster most likely what they're going to do is to buy stable coins or just get out of the market so the perfect moment to buy altcoins for the long term will come after the crash and not during it