It is just like saying can the sun shine. Altcoins have been very risky since the day they have started to be created, ICOs, IEOs, pump and dump, DeFi and many others are risky. Some are like ponzi scheme and some are just not worth it.
During bull run, you can buy altcoins, but during bear market, better to sell because they are most volatile and some altcoins will remain dead.
Buying altcoin at all-time-high is another problem because most of them do not use to reach all-time-high again. You can check many altcoin created like 5 years ago to see that I am right, which means they are not store of value.
I agree with you that During a bull run, you can buy altcoins, but during a bear market, it is better to sell because they are the most volatile and some altcoins will remain dead. But all of that must first do research on market predictions, because if we panic in terms of investing then we will lose more than profit, I don't think the market has to be Bearish anymore, we immediately sell the coin, who knows it's just a drama being played, even though if we save it might happen 10x the profit we get. And every investment in any market also has its own risks, especially investing in crypto, I suggest that you choose to invest in stable coins, such as Bitcoin or some other Altcoins which in my opinion even if there is a decline from time to time but the coin has the ability to recover after the decline i.e. Ethereum or BNB.