I don't know if this will always be true, but it has been true since the beginning of crypto: BTC doesn't rise as much as alts in a bull market and alts don't retain as much value in a bear market. If you think the bear market is bottomed out and you trade alts for BTC based on its performance, based on past markets you'll be making a mistake as the BTC you bought won't rise as much in the bull market. The smartest/luckiest people in the room sold for cash and are either buying alts now or avoiding the market entirely, not trading their alts for BTC. Also, it's a good time to hedge if you are long BTC given you are getting more bang for your buck and there's no guarantee (in fact there's a lot of doubt) that BTC will be One-Coin-To-Rule-Them-All.

The other part of this is what coins you are holding: if the outlook for the coins you are holding hasn't changed or has gotten better, there's no logical reason to sell as the price isn't correlated to future performance, just the mood of the market. If the coins look like total  shit, they were probably shit when you bought and the hype cycle reeled you into buying. I know it's tough to ignore or stifle emotions when it comes to money, but you'll have better performance if you do. As the saying goes, "be greedy when others are fearful and fearful when others are greedy."