> COVID only killed under 1%

Given that "nearly everyone" was exposed, that value is an order of magnitude too high, should you compare it with how many died the years before covid and during (but before vaccination was generally available) as fraction of the presumably exposed population. All of this is public data, but it probably makes sense to exclude countries with notoriously bad data such as China and India.

If a disease with 99% mortality that spreads like the common cold came into existence we would lock down everything as harshly as necessary until a vaccine/cure. That'd involve stopping all international flights and so on (and all domestic travel, and maybe even going out of your suburb). Covid wasn't deadly enough to justify enforcing the most drastic measures - a 99% mortality rate would justify them to almost everyone.

What if it takes 4-6 years to show symptoms like BSE?

We’d be dead.