My progression has been the 2560x1440 Thunderbolt display to the Dell P2715Q, which has been my main monitor at work and home for the past 5 years.
I've tried to upgrade from the P2715Q three times now, and always end up going back to it because I'm dissatisfied by the quality of the other panels in comparison.
I want a larger screen that can support 4K, or even 5K, with comparable panel quality. I avoided the LG 5K display sold by Apple because I heard bad things about it, but never actually tried one. It's 27" as well, so I think it would still be too small, even if the panel quality and resolution are better.
I'm starting to think I should just get a TV and use that as my display, but I don't know which would best suit my needs for writing software. One nice thing about a TV is that it would be much easier to do returns/swaps if I need to play the panel lottery for something that will make me happy. I have never done this with a previous display, since I always bought second-hand from people who had already done that for me. The original owner of my P2715Q apparently returned theirs three times before they were satisfied.
I've toyed with the idea of getting an Apple Pro display, but I really don't want to spend more than $2000 on a monitor.
The UP3218K described in the article seems like it would be a good upgrade, but it definitely exceeds my price range. I would be open to spending that much on a monitor if I knew I wouldn't be replacing it for at least five years, but I can't know that for sure. Although funnily enough, that's what has happened with my P2715Q. It's the only piece of computing equipment I haven't replaced since I first started using it.