When there are a variety of species, a single pathogen is less likely able to ravage an ecosystem.

Interesting, I just had a chat about this hypothesis with a Lyme disease expert. Lyme is apparently held up as the best example for this argument, but field data and mathematical modeling indicate that it isn't true (I could probably dig up the relevant paper if you're interested, but I haven't read it).

I don't know for sure about other zoonotic diseases in wildlife, but I don't think this is certain enough to just be stated as fact.

Your other points seem worthy of consideration, but on the whole it seems the marginal benefit of a member of this crowd worrying about biodiversity, while not utterly negligible, is small.