I agree and it's what I do. But I think there is one valid counter argument. If you have to pay for something, then there is always additional potential for loss of access that doesn't exist with free services.
That potential is perhaps even greater with domain registrars and all the other service providers involved in email services (registrar, domain host, email provider). Any issue with any of these services could mean you're not getting notified of problems and you may no longer be able to get into the admin account.
And if you try to avoid that by storing a different notification/recovery email with them (if at all possible), it opens another can of worms. My main domain almost expired on one occasion because I didn't get the renewal reminders at the alternative address I had stored there years ago. I had simply forgotten about it.
And after I had canceled my legacy G Suite account recently, I received a message from Google at one of my old recovery addresses telling me that this G Suite (Workspace) account was going to be automatically upgraded unless I log in to check some box. Only I could no longer log in as the account didn't exist any more, nor could I contact support as that requires logging in as well.
Every single account I pay for requires some sort of constant monitoring and maintainance. Otherwise, things just degrade and access is eventually lost. That's why I'm not sure whether I would really recommend everyone rent their own domain.