Insisting that a browser or device be attested by the manufacturer locks out smaller firms or indie developers from developing alternatives to said browsers or devices, as they would be in a catch-22 situation, where they'd be seen as illegitimate till they get a broader audience, and unless they attest their devices they can't get an audience.

The same situation exists in other areas as well. Trying to bootstrap a search engine? Well tough luck, because what are you trying to do with a headless version of Chromium?

This in turn creates a cartel where only few players can compete, and the members of said cartel can enter new markets without ever having to compete.