If you broke Comcast up into smaller companies, I don't think you'd suddenly have multiple different companies competing to dig ditches and lay fiber in each tiny town across America. You'd have Comcast North, Comcast West, Comcast South, and Comcast East, and the situation is literally still the exact same except that the owners of Comcast become a little richer due to the conglomerate discount.
There's a good argument that cable/internet providers are a "natural monopoly" in which case governmental ownership, or strict regulation is warranted. I.e. make them utilities.
The physical last mile cable is a natural monopoly. Internet service, i.e. connectivity to the rest of the world, isn't. So it makes sense to separate them.
Some countries have local loop unbundling or similar and it works pretty well.
https://www.investopedia.com/terms/c/conglomeratediscount.as...