Why would you expect a vaccine to confer significantly longer lasting immunity than an ordinary infection? What evidence do we have that natural immunity lasts any longer than a year?
N=1 that natural immunity lasts long has to do with my 70+ year old dad who got covid Summer 2020 and has been doing blood donations ever since as he has type O blood. Every time he goes in they report back that his antibody count is still high. I can't offer you any studies on that, just a personal antidote.
Edit guess I'll get more minus points for my N=1 antidotes
There are some notable exceptions though, e.g. HPV, Tetanus, and some others.