I recently upgraded from a 26" 16:10 1920×1200 display to a similarly-sized 16:9 4K display. I ran them side-by-side for a few days to compare. The difference in text quality was night and day. After using the 4K for a week the quality of the old display looked terrible: blurry and unreadable. Like going back to 800×600! I sold it; I saw no value in it as a secondary display. If I do ever need a second, it will be another 4K. Or 8K when it's available and affordable. If you're developing, writing or reading all day long, the improved legibility makes such a difference for ease of reading, eyestrain and headaches that it makes little sense to retain the old.

We clearly need to support low DPI displays for some time to come; the installed base is huge. However, high DPI displays are the future, and it's going to be increasingly a requirement that they are also properly catered for.

I'm not a particularly big fan of the trend by Microsoft (for example) to use very thin fonts with Windows 10. Just because you can to show off the technology, doesn't mean you should. I'd prefer bolder, more easily legible text even with a hi-DPI display, even if the thin ones are fashionable for some reason.