Aaaaaand there continues to be absolutely no viable option to have an external macOS display with Retina.
I agree that the 5k 27" display is the sweet spot for a Mac display - because MacOS doesn't have variable PPI and 5k is "just right" for MacOS's design. Apple's problem is that 5k just hasn't taken off with the PC world - there's no particular 5k advantage on Windows - hence the disappearance of 5k displays from Dell, HP, Phillips etc.
The related problem is that DisplayPort 1.2 hung around way past its sell-by date, and 5k really needs DisplayPort 1.4 to be done properly. With DP1.2 it needs two DisplayPort cables - and even with Thunderbolt it's implemented as 2 DP1.2 streams each driving half the screen. Again, with Thunderbolt not catching on so quickly with PC users, that probably helped dissuade PC users from taking up 5k.
Of course, you may not care about PC users, but the reality is that anything that only Mac users buy loses out on "economy of scale" which means that 5k panels are going to be rare, expensive or both - Apple probably need every panel that LG makes for iMacs... So if Apple do produce a 5k display, it's probably not going to be cheap.
In the absence of 5k, though:
"looks like 1080p" is fine on an up-to 24" 4K UHD display - and it isn't 1080p it's 3840x2160 but with chunky system fonts and icons - any app with a zoom feature will take advantage of the 4k. It's not that bad at 27", but the icons and fonts do start wasting screen estate. Personally, I'd probably go for a pair of 24" over a single 27".
On a 27", "looks like 2560x1440" is a bit soft compared side-by-side to a 5k iMac and, again, isn't just 1440p - its 2880p downsampled to 2160. Plus, you can switch to either 1:1 or 1080p in a jiffy if you're using an app with a zoomable interface.
Or, if your eyesight is sufficiently good that the previous solution is too "soft" then a 30"+ 4k will probably be usable for you at 1:1.