Speaking as an owner of the 4.7" 6 who's on the fence about switching: if I were you, I'd try to focus on the fact that it's part of a trade-off that gives you a higher ppi than the 4.7" 6, plus OIS and 20-40% better battery life.
I agree, of course, that it does indeed suck that they did it this way.
Edit: One other advantage for the 6 Plus is that zoomed mode takes the 667x375 points of the 4.7" 6 and then scales them @ 3x to 2001x1125, and then downscales to 1920x1080. If they had an even scaling factor for standard mode, then the zoomed mode would have likely been stuck rendering at a resolution below the display resolution. Their downscaling solution allows both standard and zoomed modes to be accommodated without either being completely crippled.