Assumptions often turn out to be incorrect. Put a pathogen under severe environmental pressure and it mutates. Given that death is the alternative, slightly harmful mutations that avoid death look pretty good.

Invariant regions are great but note that humans have not done this for colds, flu and now covid. And billions have been working on it.

The spike of the virus was always a bad target and that was known at the beginning. Some have posited that it was an "easier" target for which to create a vaccine but I have not seen compelling evidence why. Others have suggested that it was chosen because no one gets rich selling a vaccine that actually cures a disease after the first dose.

no everyone wasn't, obviously some people would have proposed that though, along with hundreds of other theories on how it would go.