I think the market is ready for 21:9, but I would split the target users into two camps:
1. Those which are COMPUTER users - the 2560x1600 format or 2560x1440 formats (16:9 ratios from 27" to 30" in diagonal) have run their course and are due for an upgrade. I think that upgrade can be in the form of a 4K 21:9 monitor (say 3840x1600 for example). A 34" to 40" diagonal should still be a good size for the 2 foot experience. I haven't looked into graphics cards supporting 4K, but that would obviously be a requirement before I buy one.
2. Those which are MOVIE lovers - same as above, would want a @4K 21:9 monitor (3840x1600) but the main difference being a larger monitor - 50" to 70" diagonal, to satisfy the 10 foot experience
The movie lover version (2.) is contingent on content, which is not yet available at 4K, but likely this will change in the next 4 years as the entertainment industry starts to experiment with delivering 4K versions of their movies to consumers, and Blu-Ray adoptions starts to surpass 70% market penetration. But 1. above is not contingent on content. Computer users will gladly use the additional resolution and desktop real-estate to display more windows, widgets and icons.
I personally watch all my movies on my computer, which doubles as my living room TV (2560x1600 @30"). This is complimented by a 7.1 audio setup (so I agree with related posts that buyers of 21:9 screens will not want - or care about - built-in speakers). Maybe my situation is unique, but I would love to upgrade my setup with a 4K 21:9 screen at, say 36". At the moment I'll continue to hope and dream.
Anyway, I wrote about 4K on my blog recently: