There are tons of expensive monitors available that cater to the professional market. The whole premise is not based in reality. Apple simply has the highest mindshare among average products.
It doesn’t matter how much money you throw at the problem, nobody is making 27” 5k displays right now.
So clearly, Apple could sell a monitor for about $1600, that would be perfectly compatible with Mac Pros, mac mini and as a secondary monitor for all the various MacBooks.
Basically, the ideal PPI of mac displays is a multiple of 110 PPI. So, for retina quality you need a display of roughly 220 PPI, which is what you get from 5K at 27 inch. A 27 inch 4K display is around 160 PPI. If you use that in 2x mode, things will appear too large. If you set it to scaled mode to make things appear the proper size, there are display artifacts (like shimmering when scrolling). In fairness, it's not super obvious unless you know what to look for. But if you're already spending money on a high end screen, why should you have to compromise?