You can think what you like, but the scientific literature says otherwise.

Diseases get less severe over time because, it turns out, the more deadly ones have a lower chance of spreading vs less deadly ones. Virus strains that inhibit the host less have a longer time in contact with potential new hosts spread faster than the ones that have severe, early-onset symptoms. So without human intervention, viruses trend toward being less severe.

Long COVID is a separate thing, any I'm honestly not that knowledgeable on it. I personally think we need a better understanding of what's going on because I'm not convinced COVID actually caused all of those cases, and maybe not even a majority. I think doctors have just been throwing the label at it when there's not a ready explanation and the patient had COVID recently. Vaccines do seem effective at reducing the chances of that diagnosis though, which makes sense since they're designed to reduce the severity of the disease.