How about the fact that in the prior 20+ years it did not happen. Comcast was just as powerful five years ago, as they are today. The access market was just as limited in terms of competition, 5 or 10 years ago.
Your premise is that due to limited competition, it must occur. If that were how things actually worked, it would have occured before net neutrality in a very big way.
Second, you are right, Comcast would have loved to turn the Internet into cable TV five years ago, and as a member of the protected oligopoly, they would be in a good position to do so, except... oh yeah, net neutrality regulations prevent that.
Third, now that we are in modern times where most people do use the internet, it already has occurred where no preventive measures exist. There are examples in this very thread; click them.
It's not fair to look to the past to make predictions about the future here because the world was very different from a business perspective.
TV is monopolized to hell and back and full of advertising people can't stand. The internet provides everything you could want, without the cancer that has infected television. People wouldn't cut TV out of their lives if TV actually provided what they want at a fair price.
Why on earth would you think this would not happen? Because the tiny handful of telecom and cable companies, already protected from competition by various laws and regulations, will decide to leave hundreds of millions of dollars on the table so that people like them more?