I read that this is a common misconception: the high heat is not enough to denature any proteins (else it would kill you too) and, what's more surprising, it actually makes viruses/bacteria more active. But it also makes your immune system more active, with an overall win in effectiveness over the microbes, which is what makes it useful.

But you do sound like an expert.

Viruses do adapt and mutate though. Look at all the various strains of H1N1 and SARS-COV-2.

Just because they don’t reproduce without a host cell doesn’t mean evolution doesn’t happen. If a trait emerges that is beneficial to future generations, viruses carrying that trait can infect more cells and spread further.

Usually it’s evolution itself that people give too much agency to. Mutations are a crapshoot. They can be beneficial or they can cause birth defects, sterility, prevent reaching sexual maturity, or make finding a mate excessively difficult. Or all of the above.