Well that's the problem, Autolayout is a freakin' nightmare to work with, especially if you want to do it visually in Xcode. It's horrible and frustrating to work with, easy to break your layout by changing anything, and difficult to debug when it's not working right, which is often. You can also do it programmatically, but that's a lot of extra code because you need TONS of constraints for an even moderately complex layout.
And Autolayout doesn't work at all for games, in general.
What I would prefer is sticking to multiples of what we already have:
2X = 1136 x 640 (current iPhone 5+) -- 326 ppi
3X = 1704 x 960 (4.7" iPhone 6, maybe 5.5") -- 416 ppi at 4.7", 356 ppi at 5.5"
4X = 2272 x 1280 (5.5" iPhone 6) -- 474 ppi at 5.5"
That makes things a lot easier on developers and would still look decent scaling "unoptimized" apps up, better than boxing it off IMO. Would be slightly soft at 3X because of the need to antialias, but should still look pretty good at that resolution as the pixels are very small, allowing for very good antialiasing.