The time frame for a booster is based on the longevity of its protection.

The flu shot (and many others) are required for some professions. Protection for the vulnerable they interact with.

Opinion: if you don’t want to protect the vulnerable in society, you should refrain from participating in it.

Aren't the vulnerable already protected with boosters? Where is the logic?

My vulnerability is that I take immunosuppresants, which makes the vaccines significantly less effective (last I heard a few months ago, fully half of the breakthrough infections of the vaccinated were in the small population of immunosuppressed people).

OK, I get your point. It's an ideological dilemma then - you'd prefer as many people around you to be vaccinated and boosted to lessen your chance of getting infected (although, even those boosted could transmit). On the other hand, those younger cohorts getting vaccines mostly to protect the others are subjecting themselves to low, but still tangible risk of getting vaccine complications. So there is some altruism at play and this is something which shouldn't definitely be mandated by the governments. (PS: It's good that very effective pills are now coming to help immunosuppressed).

Like the flu shots, there are people who legitimately (shorthand for “have a medical reason”) can’t get vaccinated.

vaccines are more effective the more people have them.