Yes; and additionally, there's non-sterilizing vaccine immunity at play. If the vaccines are highly effective at keeping people alive, but less effective at tea

Yes; and additionally, there's non-sterilizing vaccine immunity at play. If the vaccines are highly effective at keeping people alive, but less effective at teaching the immune system to sterilize it as quickly as possible, that removes some evolutionary pressure which would naturally kill off more lethal mutations. In other words, a more lethal "viral payload" mutation could have its lethality kept in check by vaccines which are good at targeting the spike protein, until a second mutation comes around on that spike protein which better escapes the vaccines; or, simply, until it transmits to a population that is undervaccinated or has waning natural or vaccinated immunity and isn't getting revaccinated.