The PPI is only 2560 x 1440 on the Thunderbolt Display, which was great in 2011.
But now that I've seen what's coming out this year (thank you CES 2014) I'm getting very excited about the possibilities of Ultra HD 3840 x 2160 IPS panels.
If Apple was smart, they would announce something very soon, because people can only wait for so long. But I don't think anything is going to happen.
Of course you're right.
Just saying as someone who thought exactly the same, I'm sold to the actual quality that the TB display offers today! It will be a while before apple supports a proper UI scaling method for UHD displays I suppose. And when that's the case and 60hz 28" UHD displays pop up, I'll be the first who sells his TB displays.