As a CEO, you want low prices and innovative products from your suppliers. You will get neither in the network business if AT&T buys T-Mobile. That deal will trigger the inevitable merger of Verizon and Sprint -- limiting your choices and slowing the development of new offerings.
We've seen this movie before. The US car industry devolved into a few players, and decades of decline were the result. A few years ago in a post, I asked the question: "Why can't America build a great car?" Bob Lutz, a 40-year car executive, answers the question in his vinegar-soaked book (which I recommend).
I'll take license and distill his answer: The American car industry faded because of poor leadership and management. Lutz blames the failure on a culture of over-planning and bureaucracy. Spreadsheets were valued over common sense, customers were ignored, and too many over-educated over-analyzers focused on arcane process and rules rather than the job at hand: building great cars that people want to drive. It's not the unions' fault, it's not bad workers, it's not government regulation, it's not healthcare costs. It's leadership.
Which brings me back to the network business. When markets are reduced to a few players (as was the US car business), you run the risk of under-diversifying management thinking. In nature, a lack of diversity increases the risk that a species will be wiped out by a single infectious disease -- as almost happened when the group think pathogen ravaged Detroit.