Apple pulls off some amazing pricing stunts on occasion. Once you upgrade the storage and CPU of the regular 2560x1440 27" iMac to match the 5K versions, the 5K screen is only a $300 premium. They can likely afford to pull off the minor premium for 5K because their regular models do have a price premium over the 'race to the cheapest component' PC market.
If they pull off another 'minor premium' to go 8K, it will sell. 8K will be a harder stunt to pull off though, because it will need quite a beefy GPU to go along with the screen, which is tricky in the ultra-slim all-in-one form factor.
8K is like having a 4x4 grid of 1080p displays. In a few months we'll see if LG can squeeze that into 27", or if we get an 30" iMac or something instead. The extra size allows for a lower pixel density, but also more space for cooling/heatsinks. And once it's 30" they can attach a bigger premium, even though it made things easier on them.