The largest email providers already show ads to the free tier. They don't earn that much per user and it's just not worth supporting custom domains at that scal

The largest email providers already show ads to the free tier. They don't earn that much per user and it's just not worth supporting custom domains at that scale. If they cant make it work, why would anyone else have more success by offering even more features for free? And if you're already buying a domain name then why not just pay for the email account too?