Not so sure about this. We still mostly have laptops with shit 1366x768 monitors despite "retina" or at least "full hd" screens being around for quite a while...
I bet it's a bigger deal for external monitors. A ton of people buy laptops just saying "Oh, 15 inch screen for cheap, that sounds perfect!" But the sort of consumer who buys an external monitor tends to know what "resolution" means. There's demand for better ones because nobody buys a 27" 1366x768 display.
Given that Dell's 5k 27" screen is priced similarly to these iMacs, I think the economies of scale could help push some prices down.
I feel like laptop resolution progress stalled in the early 2000s. Maybe mass knowledge of 1080p HD set us back. I guess consumers started thinking "1920 x 1080 is really good resolution" so they were willing to accept lower resolutions as "good enough" for a long time. I had a 1400 x 1050 14" Dell laptop in 2005 but it wasn't until the Retina Macbook Pro in 2012 that laptop resolutions got significantly better. 768 is an absurdly bad vertical resolution. Many serious content creation apps do not work well at all at such a low resolution.
Blame Intel for that. When Intel's chips represent a whopping 40 percent of the machine's BOM, there really isn't that much room left for a high-resolution IPS display.