I don't see any evidence, just handwaving.
It sounds like you don't understand how herd immunity works. The idea of herd immunity is to reduce the average number of people infected by one infected person to less than 1. If that is achieved, then the disease cannot propagate even if introduced, and peters out. As a result, the chances of anybody coming into contact with the disease become tiny. Note that vaccine protection doesn't have to be perfect for herd immunity to work. The probability of breakthrough infection can rise with time after vaccination, but so long as it remains lower than for an unvaccinated person, it contributes to herd immunity. Moreover, even if a vaccinated person manages to catch the disease, which means that they tend to have a less severe infection of shorter duration, so the likelihood that they will pass on the disease if infected.
So a decline in serious complications of the disease with time is exactly what I expect, and it's exactly what the statistics show. It's been about 18 years since the vaccine was introduced in the US. So where is that spike in adult hospitalizations and deaths?
Now where is your evidence? Or is uninformed hand-waving all you've got?