Anecdotally I've seen reports of people getting Covid more than once. So even the assumption that if Delta runs rampant through the unvaccinated population at least we'll gain full herd immunity that way is perhaps not true. Plus you always have people too young to be vaccinated or immunocompromised that will be a reservoir. I'm not an epidemiologist and claim no expertise but it certainly appears we may always have the threat of this virus in the background. Bleh.