It does diminish. The vaccine infects cells, then the infected cells keep pumping out spike proteins until those cells die. Those spike proteins train the immune system, which remembers how to fight them theoretically forever, but in practice the training is shitty and also doesn't last forever.

The amount they pump out diminishes with time, but it diminishes for everyone even if each persons rate of cell replacement is different. It's supposed to happen in a couple of months, but it's a spectrum and in some people we've seen them pump them out for a year+.

But basically everyone will be clear within a few years and if you make it past the first year, your cells aren't significantly compromised anymore. You've still got a shitty trained immune system, but you won't keep shedding spikes or worsening your immune system.

You've suffered whatever damage dealt from the spike proteins (eg, to the heart), but you aren't taking any more damage.