An argument against custom domains being a "premium" feature, then? If you don't have a paid plan, then you have to bring your own domain. Paying gets you the *.example.com status symbol. Makes sense — a custom domain usually requires technical expertise and some amount of inconvenience for users (not unlike a services company that publishes their source code to GitHub, which you can always set up and maintain for yourself, but if you want to use the company's hosted services, then you have to pay). Another part of the value proposition would be, "You're gonna have to spend money one way or another — either to a registrar or to us — so why not let it be us?"

People trust few domain extensions more than others even though many of them are similarly priced and available. It's all about who else is using the same extension. .com? Used by most companies. .news ? no one? not trust worthy then.

It's somewhat fascinating to me that the bigger the company, the more trust people put in their verification of actors employed on the platform. Intuitively, I would think the more actors in a company - the less trustworthy or verified the actors would be.