T-mobile's US assets are worth a significant amount, so they're unlikely to simply shutter and go away. Think about this regulatory hurdle not as dooming T-mobile, so much as sending their assets to some company other than AT&T.
I wasn't upset at all about the merger (quite the contrary), but I don't think the outcome is as bad as you say it is.
But if AT&T doesn't buy Tmo, one of these might merge instead with Tmo or buy those assets directly.
At other times, there has been speculation (or even work done) on a Sprint / Tmo merger which would bring two weak players together but also allow for more direct competition between the GSM and CDMA world with an LTE path going forward.
They use completely different technologies with incompatible handsets and cell towers, so a Sprint/T-Mo merger is likely to be worth considerably less together than separate.
I wasn't upset at all about the merger (quite the contrary), but I don't think the outcome is as bad as you say it is.