While I agree with your suggestion (that's what I've doing for years), I'm not sure the "almost not effort" point is helpful or realistic when it comes to a huge majority of users.

Apart from people who are knowledgeable or at least curious enough to search for alternative options, I suspect many people don't even know what a domain is or that they could register one for them to use. The jump from "why pay for email if Gmail/Outlook is free?" to "register a personal domain and use it for your emails" is too big.

I don't have the ideal solution but what I've suggested to friends and relatives is they should consider paying for their personal email accounts. Most of them don't care but some do and, as a result, at least try to understand what they want or need and are willing to pay for.