I feel your concerns, y'all... However I can say that WP 8.1 has been subtly, but heavily, optimized for 1080p displays.
The new hub design within core apps and, eventually, third-party offerings is going to look proper and sophisticated. The header text has been shrunk significantly, meaning the People and Music hubs won't be dominated by the words "People" and "Music+Videos." A good 2" is taken up by the headers on my 1520!
Speaking of the Music hub, did anyone notice the player controls on the Now Playing screen are now on the bottom! This is gonna be a
huge usability improvement on my 1520.
I have confidence that this update will make our 1520's feel right at home..