I don't see it as "zero sum." We old folks are, after all,




sapiens. We share the same basic biology.
You're right though, that vaccines don't get tested on the very vulnerable - but there are tons of studies using data from the fairly vulnerable to inform us.
There has never been a vaccine with the kind of pattern that you describe (99.9% safe for people under 75, but "myriad complications" for the same group of people, but 1-20 years older).
It doesn't work that way. A vaccine challenges the human immune system in very predictable ways. We approve them for 6 month old babies (!) based on trials with older people. They work.
The test groups of the most of the current vaccines do include older people - so we *do* know the effects on some older people (but not children). I'll be happy to be part of the older trial group, as I am very clear in my mind about how vaccines work and what constitutes a vaccine.
But I do want my vaccine type to be tested in a broad range. For me, the fact that the Oxford Vaccine (as an example) was initially tested by a group 18-65 is good.
The reason that older people have less effective results has nothing to do with the vaccine and everything to do with how T-cell immunity works.