Sure, but changing the resolution and changing the scaling means changing the performance metrics. If one device is 1080p and one is 2160p, then even if both "

Sure, but changing the resolution and changing the scaling means changing the performance metrics. If one device is 1080p and one is 2160p, then even if both "render" at 1080 and one scales up to 2160 that is a change in performance. They don't want developers to have to test on multiple devices to see if it gets laggy on the one with the higher-res screen.