You have like a 120+ degree feild of view without moving your head, so if your getting a sore neck your monitors are either pretty much on your face or they are at the wrong height.
As for 5k on 28"… there is little point for most tasks. You won’t get more text or veiw more of a project (be it a photo, video or CAD) on screen due to such a high pixel density, it’d just be too small to see\read, you can’t really find much consumer content >4k to play back and running games at multiple 5k displays is graphically prohibitive. Thus you may as well have a larger monitor, its displaying all the same detail, but doing so large enough its quick\easy\low eyestrain to see. Nothing beats working on a video where you can tell scenes apart on the timeline without scrubbing through it, you can see all of the settings of the thing your currently tweaking without scrolling up and down madly and you have a live playback window as big as a normal 24" monitor without moving or full screening it. That effects productivity a lot, but having twice the pixels (8k) on that same 50" monitor would likely only benefit the few things where maximum detail is needed in the playback window.