AT&T's threat assessment of T-Mobile was correct at the time.
I think that assessment was obvious to everyone at the time. The question is whether buying out competitors is good for the public.
Of course, the cash was a penalty for not being able to pull off the merger; if the cash was critical for T-Mobile to become the threat it has been, the outcome is ironic.
The equivalent for this merger would be something like Minecraft and Bethesda games on the A-B launcher for 7 years. Huge giveaway by AT&T I think, as foolish as it might have been for them to think the merger would actually go through; having at-least 4 major carriers was policy at the time and still is (Dish's spectrum hoarding notwithstanding).