More effective than breaking them up would be imposing regulation. Force the incumbents to allow 3rd parties to use their infrastructure at a regulated wholesale rate, and then compete on service.
In Canada, there's a bunch of smaller cable resellers. For instance, Shaw does cable/phone/tv and also has some wifi infrastructure deals with various commercial businesses, and provides "Shaw Go Wifi" access to internet subscribers.
In my local area, we also have LightSpeed which resells access to Shaw for a cheaper rate, but you need to buy your own modem, and there's no wifi access around town. Trade-offs, and a slightly more healthy marketplace.