I know. What's happened sucks and I wish ND we're able to leave supersampling anti-aliasing (SSAA) for 4K mode as was, but what other explaination is there? That ND dislikes 1080p TVs so they removed all SSAA? That Sony made ND do it to encourage sales of 4K TVs at the cost of angering a lot of people? Or that ND needed a quick fix to the performance problem and that fix happened to be removing SSAA?
I don't know what the cost of the downsampling part is but to render a game at 30fps means an image needs to be produced in 1/30th of a second. According to this presentation by a developer named Tiago Sousa (
http://iryoku.com/aacourse/downloads/13-Anti-Aliasing-Methods-in-CryENGINE-3.pdf), the time budget for anti-aliasing in Crysis 3 was only 2 milliseconds (1/500th of a second). They started using a technic called temporal AA that supersamples from past and present frames rather than trying to supersample each and every frame as it produced better results than FXAA but was "cheaper" although not quite as good as SSAA or MSAA. ND have a great implementation of temporal AA for Uncharted 4 (
http://www.eurogamer.net/amp/digitalfoundry-2016-uncharted-4-thiefs-end-tech-analysis).
Anyway, point is that if the budget for anti-aliasing is 1/500th of a second it'd only take five frames a second to exceed this limit and the frame-rates will have dropped from 60fps to 55fps. Most Pro games that are rendering in 4K don't use any anti-aliasing as the high resolution means it's not needed so on 4K TVs there's probably no AA while on 1080p TVs there's no longer SSAA but something else instead.
I sound like I'm supportive of what ND did but I'm not, just trying to figure out why they did it. I'm all for the return of SSAA to TLoU:R!