Who gives a damn if the UI is at 720p, 1080p or higher?
It's not like you sit on the couch staring at the UI for hours.
Whilst it's certainly a tertiary element of a system, I've still spent a fair amount in mine, eg looking at screenshots I've personally taken, or that have been shared in messages, plus videos, editing videos, transferring files, sending and typing messages, arranging or managing parties, browsing the store, libraries, storage options, looking at content and streams from other users, checking trophies/achievements, using integrated apps, searching users from LFG groups or recently played to add in parties and so on.
But again, if a 4K UI is really going to cost 1GB of extra memory vs the 1080p alternative, and developers are asking to use it for games, so be it.