Right. Software diversity (as long as it's real, all-the-way-down diversity and not just different branding of the same tech) is beneficial to overall security

Right. Software diversity (as long as it's real, all-the-way-down diversity and not just different branding of the same tech) is beneficial to overall security for more or less the same reasons that gene pool diversity is beneficial to the survival of a species. This is one instance where our choice of metaphorical vocabulary, like "virus", is very apt. Only one vendor for all corporate email is bad for the same reason that only one popular variety of banana is bad.