There is really no such thing as too big to fail. In the case of the "financial crisis", the few failing banks should have been allowed to fail. Some were and the sky did not fall. There were plenty of sound ones who were not involved in the nonsense. The same can be said for the automotive industry. The bailouts were nothing more than a transfer of wealth orchestrated by the government. From the middle class and the wealthy, to the incompetent clowns in the government and private industry who created the mess in the first place.
There is absolutely nothing worse for the people at large than the collusion between government and the private enterprise. It is very simply the definition of corruption. Read the Wealth of Nations by Adam Smith for more love.
As for AT&T and T-Mobile, the government created the problem in the first place. By selling spectrum, they necessitate the existence of large massive companies in the space. They're the only ones who can afford the spectrum. If the government instead chose to keep the spectrum open, like say the way they do for WiFi, you would literally be able to start your own cellphone company in your garage ... and there would naturally be quite a bit more players in the space. They would still need to cooperate to make things work, but having more choices is the goal here.
If you look hard enough, behind every market problem and failure, you'll find a fat bureaucrat and fat "business man" working together to keep each other fat and the rest of us miserable.