One could almost say that they did release a TB display. It has a 5K resolution and costs $2500, cheaper than the Dell version. You just have to use the iMac in Target Display mode.
Except that Target Display mode is not possible because TB 2 is not fast enough to support a 5K video stream. And combining two TB ports also doesn't help because the two TB ports share the same controller. And why do they share the same controller? Probably because the CPU and chipset don't have enough PCI lanes to provide the bandwidth for it.
And for Macs being able to drive a 5K display, at least theoretically, only the Mac Pros have more than one TB controller.
Exactly. My 2013 15" MacBook Pro can't drive 5K, but it can run a 4K display. I wanted a 23/24" 4K Thunderbolt display for 1080p HiDPI mode so bad. More than anything Apple announced today.