You just point the MX records of your custom domain to a different provider, you mean? The vast majority of Gmail users do not have custom domains, and directly give out @gmail.com addresses to other users and services.
The thing is, Google Workspaces base plan is only $6/user/mo. If you value your email (which, as annoying as email can be, you should, since it's often the key to all of your other online accounts), $6/mo is... intensely reasonable. (And you get other stuff with that too; it's not just email.) Unfortunately, we've been trained to expect that everything on the internet is free nowadays, so even that sounds like an expensive service.
But agreed, most people just have @gmail.com addresses, and thus no portability. Google of course knows this, and that's by design. I wish we had a popular email service that would register a domain for you as a part of the signup flow. That would be pretty cool.
You can host them at apple if you have iCloud+, the cheapest of which is 0.99.
MS charges you about half of Gmail (through resellers) if you don't want the office programs installed and about the same if you want that included.
Zoho mail is about 3 dollars a month and then there is that service I can't remember (just email?) that will let you host the entire domain for 10 a year.
Neither of which is very expensive.
Most of the 18% are them not being bound to any contract. I am not sure I have ever signed a contract that binds the other party (I don't even think my offer to buy a home completely precluded the seller from backing out), so it doesn't extra worry me. Having a backup and a changeable address assuage any of my fears. (I feel the same way about cloud providers. If GCP nukes me, it will be an annoying day to get going on AWS, but hardly the end of the world.)