I think that is a uselessly reductive interpretation of what secure boot is because you could apply the same logic to any security technology. Why should we allow login passwords or user permissions or disk encryption, since those could be used as lock-out technologies by manufacturers, if they just ship them with defaults you can't control?

Manufacturers don't need any user-facing standardized controls to implement lockouts. So the possibility of a feature being used as a lockout shouldn't be a justification for taking away the option of having a user-controlled security feature. Taking it away from users isn't going to stop manufacturers from doing it anyway with proprietary technologies instead.