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.