Supporting both is part of the problem unfortunately. In principle I agree on how the user agent should behave, in practice leaving viewport as default doesn't

Supporting both is part of the problem unfortunately. In principle I agree on how the user agent should behave, in practice leaving viewport as default doesn't necessarily match what the user wants or what the developer wants. E.g. user zoom settings are still applied after the viewport scale setting, just now in a consistent way across mobile devices.