Wouldn't this kind of reasoning apply to every new virus, including new flu viruses that pop up routinely? A new virus pops up and appears to be just a routine

Wouldn't this kind of reasoning apply to every new virus, including new flu viruses that pop up routinely? A new virus pops up and appears to be just a routine flu, but technically we can't know about its long term side effects until many years later. So why don't we apply this same reasoning and lock everything down routinely whenever any new flu virus arises? It seems to me that you're right, we can't know for certain until many years later, but in the meantime the world goes on, we can't all just cryogenically freeze ourselves. The best we can do is make educated guesses based on previously existing coronaviruses.