I'm sure fastmail are great folks but if everyone who feels nervous because of Google's shithousery jumps ship to fastmail it doesn't really solve the problem of overreliance on a single provider.
That itself makes up the difference.
Admittedly, the more important part is maintaining your own custom domain, so you can switch email providers with a simple DNS update.
What OP is missing is that the "single provider" is about bundling dns, email, hosting, etc together. If you use Fastmail by itself and something happens to Fastmail you can change your email provider pretty easily. I do regular backups of my email anyways, so I'm not locked into Fastmail at all.
Using GMail and having your domain registered with Google means that if something bad happens and Google locks you out you are simply screwed with no recourse except running a PR campaign to hopefully get someone's attention. Even pretending like Google and Fastmail are the same (they aren't, Fastmail actually offers real support) these are still very very different scenarios.
In these discussions it's not really about the volume of people using a single provider, but about individual users relying on a single provider to perform multiple functions.
So the issue of over-reliance on a single provider is usually in the context of that provider being in a situation to take offence at something unrelated and apply penalties that impact other things you actually rely on.
For example Google don't like something you do related to YouTube or AdSense and they kill your account. Now you've got no email and your domain expires as you can't pay for it.
In these kinds of cases it exactly solves the problem as the vital stuff is not affected by shenanigans with the everyday stuff because you've spread the functions across multiple providers.