The problem is the level of trust. I don't trust Google, as an advertising company with a side business of technology, to keep its paws off of user data even in "anonymous" form. That's not malice; it's just temptation. There's way too much to gain, from Google's perspective, and comparatively little to lose by not keeping that wafer-thin wall intact.

FastMail, et al, alternatively aren't primarily engaged in the advertising business so they'd see a very small return from violating that trust and massive losses, so the gain/loss relationship is inverted.

What scares me most about gmail is the fear that one day an algorithm will decide I'm doing something wrong and terminate my account and no prospect of appealing the decision.

Do you think that Google would intentionally violate their contracts with various huge companies (Broadcom, BBVA, Colgate-Palmolive, etc) gain a little bit more advertising revenue? If nothing else the personal lawsuits for securities fraud against executives would be a huge deterrent.

I strongly doubt that they would, but it should be noted that them doing so would be extremely difficult to prove, and even harder to identify in the first place.

I see no reason why Google would treat various huge companies the same as my one person setup even if we all nominally use the same product (and anyway I'm pretty sure Broadcom et al. haven't signed up using the same web interface I would use).

Google, Amazon, Microsoft and all the other major SaaS providers certainly have strict controls to prevent misuse of customer data. Pretty much all companies end up signing up for products the same way, at least with the companies I've worked for.

You are vastly overestimating how much that information is worth to Google. Their average revenue per user is a pittance compared to the $5 per user per month you’re paying as a GSuite customer. They’re strongly incentivised to make sure your data isn’t misused in any way. It’s not worth jeopardising hundreds of millions in future revenue to show slightly more relevant ads to a handful of folks.

The strongest endorsement I’ve seen for GSuite is that even direct competitors to Google have no issues using it. They trust Google with their data that much.