AT&T was broken up into a bunch of smaller regional monopolies that did not compete with one another. (Almost a century after its founding, by the way, and decades of on-and-off litigation.)

The cable industry has evolved into a bunch of regional monopolies that do not compete with one another.

The effect of breaking Comcast up AT&T-style would be... a bunch of regional monopolies that do not compete with one another. And then, as with AT&T, proceed to consolidate via M&A into larger regional monopolies... that do not compete with one another. Same as now.

Welcome to the American telecommunications industry.