IMO, AT&T was banking on merging with T-Mobile in order to get the extra spectrum required in some markets to be able to handle the data growth. Spectrum is finite. They can build as many towers as they want, but if there's no available spectrum, those towards can't broadcast. As for people saying "AT&T has billions of dollars in unused spectrum", this is true. However, it's not spectrum in a frequency that any of the current 3G phones being sold can use, so it's pointless in that regards.
While they could have done what you suggested, I honestly believe (based on the billions of dollars that they said they've give T-Mobile if the merger fell through) that they thought the T-Mobile merger was going to be a done deal.
Obviously, that merger didn't happen. That was three months ago. Why AT&T continues to offer the unlimited grandfathered data plan since December 2011 (when they knew the merger was off) doesn't make sense, but I think it's hard to make the point that for the last four years, they were purposefully bait 'n switching.