Sadly, it basically never happened. There was the LG display that came out a couple of years later. It didn't have great reviews, and it was like two thirds the cost of an entire 5k iMac.
It took Apple over 7 years to release their standalone 5k display, and there are a few other true 5k displays (1440p screen real estate with quadruple-resolution, not the ultrawide 2160p displays branded as "5k") on the market now with prices just starting to drop below 1,000 USD.
Unfortunately in that time I've gotten used to the screen real estate of the ultrawide 1440p monitors (which are now ubiquitous, and hitting ridiculous sub-$300 prices). As of now, my perfect display for office work (gaming, video/photo work, or heavy media playback are different topics) would be 21:9 with 1440p screen real estate with quadruple-resolution—essentially just a wider version of that original 5k iMac display.