Fair enough. I HAD forgotten about the P3 color gamut of the screen on the 2015 model. It's nice (I've seen them in person and the difference in colors was definitely noticeable), but I wouldn't call it a "much better screen." More like a noticeably improved screen. The jump from 1440P to 5K was huge. It spoils you to such a degree even 27" 4K (what I'm using now 🙁 ) seems just "acceptable" in comparison. Compared to that, the jump to P3 is more like "oh, well that's nice."
If we're truly going to go with iMac lives or dies by the screen (which in a lot of ways it does) I'll take my wider color gamut with a dash of OLED, some proper HDR support, and if possible an ultra wide option before I call it a "much better screen."