From reading around (not a biologist), I get the impression that there's some kind of super-linear immunity building, not just waning protection. Standard children's vaccines often have a few doses, and not evenly spaced.

Soo... you're building up better immunity against the "baseline" virus with each dose. That immunity translates to some (reduced) immunity to the variants. But if the "baseline" is high enough, you can afford some drop in efficiency.

Perhaps in some ideal world, the boosters would be tuned to the variants. But as you say, they would need to be re-tested etc. Cf. the AZ vaccine, which through some arcane process causes (extremely rare) blood clots, it's probably hard to verify ahead of time a modified vaccine wouldn't suffer from such side effects.