Hypothesis: This feature is actually a very serviceable way for a small business or individual to have a branded email address on very cheap email hosting, while getting Gmail features for free. Google wants such people to be paying for Google Workspace if they don't want to be advertising the Gmail brand on their address.
Doesn't Google Workspace start at $7 per month? I can't see a business user going to those lengths to save $1 or $2 per month.
I'm retrieving POP emails from multiple domains. To migrate to Google Workspace I'd need to pay like $40 per month... and a couple of those inboxes rarely receive any emails.