I think this distinction is a bit blurrier than you suggest. For my personal email I pay for and use Fastmail. I think it's a wonderful option and I'm more productive in it. However, when starting a new company recently, we ended up use Gmail because it's required for many SaaS tools. Sometimes you can use something other than Gmail, but it's painful, and in other cases you really can't at all. And that's before you get into the ancillary services like calendar and contacts -- very few tools working with either support either CardDAV or CalDAV.

I'm genuinely curious, what SaaS tool requires Gmail?