If they choose to republish it, I hope they do that too. While I don't mind "leaving the DM room for expansion", there is a point where it gets ridiculous. If it's just a re-hash of the original 2e product, then forget it. I'm not into regurgitation.
Do it right, or don't do it at all.
(UMT is only 9 levels. IIRC, there are "at least" 16 sub-levels. So far, levels 1-3 and 5 [Level 5 is in the FR Interactive Atlas] of the main levels have been mapped and 6 sub-levels have been mapped in "official" FR products.)
I, too, would pay a hefty sum for an expanded *and detailed* Undermountain. (I would pay nothing for an update, however.)