Yep, no real reason to add new towers in most areas. TMo's network is spaced for AWS at this point in urban/suburban areas so they have their pick of where to put 600 panels. Merely swapping 700 for 600 on the sites that have 700 would be an immense improvement.
Likewise, if M-MIMO n41 is as good as folks say it is, AWS LTE spacing should be sufficient for that as well. Particularly since TMo can pick L-Sprint sites if those are better. Depending on area, those sites are spaced for either PCS or B41.