That is not how things actually work. First, many people with relevant "underlying health conditions" don't know they have them yet. The public warnings talk

That is not how things actually work. First, many people with relevant "underlying health conditions" don't know they have them yet. The public warnings talk about visible things (obesity etc) because they are the only ones it is useful to publicise. It doesn't mean they are the only relevant conditions. Second, Covid-19 strikes down many healthy people, and "long covid" is an awful long-term disability. Third, vaccines aren't perfect at an individual level. Their use tips the balance to the disease dying out, but they don't confer perfect protection to every individual that is vaccinated. People still depend on each other, so herd immunity still matters.