> The typical argument for vaccines is much less shaky than the typical argument for pure FP.
I suspect you aren't that knowledgeable about either.
> For one, vaccine people can actually explain why vaccines are good.
Unless you have a very particular background, I'm suspicious that you can actually follow the frontier of that argument. More likely, you're getting the ELI5 explanation.
> FP people seem to mostly just repeat variations on "it's easier to reason about" and showing trivial functions that are generally not easier to reason about.
Well-designed FP languages reify coherent low-complexity formal semantics. When FP advocates say "reason about", they mean in a (semi-)formal sense. No popular imperative language has any kind of denotational semantics, so good luck reasoning about the compositional behavior of C++ libraries, unless your definition of "reason about" is "think about informally".