That’s not a hard rule and there are plenty of successful viruses that are very dangerous. Bubonic plague, Smallpox, chickenpox, mumps, are rubella just a few examples. We managed to mostly eradicate them via vaccination campaigns, so people don’t remember how horrific these viruses are nowadays.

Omicron is already incredibly dangerous because there’s roughly one in six chance that you get organ damage, cognitive impairment, and chronic fatigue lasting for years. Combined with the fact that you can get reinfected, that makes it only a matter of time until you end up with a serious chronic condition.