It befuddles me somewhat why some people would prefer worse frame rates and less immersion just for the sake of better image quality. PC gaming is a medium of MOTION and the frame rate and immersion factors are HUGELY attributable to the enjoyment factor. Not that image quality isn't, but once you recognise that the graphical demands of 4K are so much greater than 1440p UW and the performance hit you will take, there's just no debate in my mind.
I am an X34 owner myself and could never go back to 16:9 now. Whenever I do sit in front of a regular screen I feel like something is missing. I have seen 4K and yes it looks nice, but no way would I ever trade my X34 in for one. I might consider it at 40"+ where I would not be losing any of that immersion factor (a 40" would be the same width as a 34" UW), but I'd still have to contend with a huge drop in performance. The fact is that there is just no single GPU capable of pushing AAA games to the max in 4K. Not so much of an issue if you play older games, but I doubt many gamers would say they have no interest in any future games and are happy to forever game on older titles. My X34 however, copes quite well with a mere 1070 in most situations, albeit not always maxed out but it still looks great. Crank up to a 1080Ti and you're laughing!
My advice would be to get a 1440p UW and experience it... if for whatever reason you feel it's not doing it for you, then you can try 4K, but I'm not sure how that will give you what you find lacking at 21:9, unless ALL you care about is image quality. The only drawback with 1440p UW, as you've highlighted, is the high price... but it's worth it IMO.
