Apple have been demo-ing the Mac Pro's 4k capabilities with third-party displays for 3 years, and the current Mac Pro really is targeted at graphics/video pros, who are going to want a choice of displays to suit their application. There are Pro displays out there that make the TBD look cheap...
Part of the point of the Mac Mini is that you can "Just connect your own display, keyboard, and mouse" (first paragraph on the Mac Mini web page) so a $1000 display optimised as a laptop dock isn't really relevant.
All Apple need for the Pro and Mini is something from a 3rd party that doesn't say "Dell", "HP" or "Lenovo" in embarrassingly large letters on the front.
The TBD is principally a laptop peripheral for the well-heeled consumer. The current display will likely be discontinued when MacBoo Pro's with TB3/USB-C launch (since they'll need a $100 adapter to use it and might not even have magsafe). They might replace it - but they might also wait: the current retina MacBook doesn't support thunderbolt and Intel's current TB3/USB-C controller can only support 5k on a Thunderbolt display. What might make more sense is a ~21" 4k display (like the 4k iMac) with a displayport-over-USB-C interface (which can also offer docking facilities).