We could look at the flu for clues, in that case every year will have one or more variants that are the most prominent.
I'd argue that in the age category with the most infected (10-29 years old) in the UK, that these people are not killing themselves due to their young age.
And the elderly have already gotten the vaccine and are thus up to eight times more likely to not get infected.
By contrast SARS-CoV-2 only has a single chromosome, and so it primarily evolves via "genetic drift". While it's _possible_ for a cell infected by multiple SARS-CoV-2 variants to generate "chimeric" offspring via a process called "crossover" AKA "homologous recombination", that mechanism is much less effective than the simple segment-reassortment that drives flu evolution.
Like the 1918 H1N1 second wave mutation that caused ARDS in 20 year olds?
We still have no idea how that happened, but with this coronavirus evolving to be more transmissible/virulent that is still on the table. If it can evolve to become more transmissible but at the cost of triggering ARDS like that, then it will face evolutionary pressure to do so.
And hospitalization rates of 20-29 year olds aren't anywhere near as good as the fatality rates. While the death rate drops by a factor of 2 every 8 years, the hospitalization rate drops by a factor of 2 every ~15 years. And delta has 2x the hospitalization rate of alpha across age/sex/deprevation/comorbidity matched controls. It should still be about a 1-in-50 chance of hospitalization for 20-29 year olds. There's no proof that number can't continue to get worse.
Not necessarily the case. Infections with HCoV-229E produces durable immunity and reinfection is due to antigenic drift:
https://www.biorxiv.org/content/10.1101/2020.12.17.423313v1
That it at odds with prior studies that found ~12 month reinfections with coronaviruses, but it is with considerably better methodology.
And it is simultaneously: not clear that vaccines produce sterilizing immunity, not clear it is necessary to do that at all, and not clear if how much measurements of NAbs correlates with sterilizing immunity.
I can tell you a story about how delta reaches 90-100% thresholds of herd immunity in this wave and burns itself out and we don't see it much again for 3-5 years in countries like the US/UK. Or I can tell you a story about how it keeps coming back every year with 2 or 3 waves. Anyone who tells you they know which of those outcomes must be correct probably knows a lot less about virology than they think they do (even if they have a degree). I'm reasonably hopeful its the former, but I never predicted delta and thought we'd be virus-free this summer and was wrong.
Even in the worst case though the vaccinated/recovered are never going back to 2019 levels of susceptibility to severe outcomes, so we're looking at a steady drip-drip-drip of people killing themselves to avoid vaccination up until we get bored with that and they pretty much disappear (that's where T-cells and B-cells and cross reactive partial immunity to all future variants comes in).