5k is pretty much an orphan resolution - I don't think you're ever going to see it take off again,
It
might catch on if DisplayPort 1.3/1.4 take off so we can have SST 5k displays. Recent graphics cards have DP1.4, but it wasn't an option when the current crop of 5k displays were released (and Intel still don't support in in their iGPUs or TB3), so we have this MST/dual cable malarky. Even TB3 uses MST for 5k - it just combines 2 "virtual" DisplayPort cables into one TB3 signal to get them down the wire - and on desktop PCs, getting the output from a PCIe graphics card to the TB3 controller is an unnecessary kludge.
Possibly, by releasing the iMac 5k, Apple made the industry jump the gun a bit...
However, yeah, at 27-30" 4k + scaled display mode ("looks like 2560x1920") is perfectly good enough without the extra hassle of 5k. If you're fussy enough to want your video previewed at 1:1 you'll probably also appreciate seeing it on a separate dedicated display without distracting frames and controls and (possibly) with different brightness/contrast settings & at a sensible viewing distance.
and I'd be willing to bet that given Apple are planning on making a "pro" display, LG will discontinue their 5k display without ever releasing a second model.
Since Apple's new "Pro" range seems to be firmly aimed at the "if you need to ask the price you can't afford it" end of the market it would be unsurprising if they went straight for 8k with their "Pro" display, rather than risk cannibalising other products.
However, what the story is behind the LG 5k is, I don't know - but the limited array of ports and even their positioning is so much like the original TB display and iMac that I wonder if the
innards were originally designed for an Apple-branded 5k TB display? Stick those innards in a nice iMac-style aluminium case with the panel bonded to a flat glass front and (a) bye-bye EM shielding problems and (b) bye-bye ugly "forehead" to house the camera. Just a theory...