I'll just leave this here, but it does need a little updating. It shows how little regulations have had an effect.
The regulations did have an effect in the 1980s, of course. That was back when there was only one company that did long distance service, and (earlier) you rented, not owned, your home telephone. So long-distance calls were very expensive, and a Big Deal, and you had only one or two phones in your house, because they were so expensive.
https://en.wikipedia.org/wiki/Media_cross-ownership_in_the_U...
For example Dr Oetker owns half if the European food industry
It would seem so since there must be an active body to oversee corporate control.
At&t needs to be broken up by region again. Along with Monsanto, Walmart, Comcast, Verizon, Google, Facebook and Microsoft.
That's not too say nothing should be done to prevent abuse of the control they do have.
The only ones on your list that create problems would be Comcast and Verizon (assuming not wireless) as they are granted local monopolies by local municipalities and governments through right of way access to utility poles, etc. Breaking up the others, as you are suggesting, would only harm consumers.
Your own example, Bell System, is an example of an sub-optimal but obviously adequate regulated monopoly that worked moderately well over a century.
And US Steel certainly did _not_ have competition when anti-trust was brought against it. By 1907 there were no longer any competitors and anti-trust attempts only started in 1911.
What you do have today, for adopting pro-monopoly policies for decades is, among other things, the highest bandwidth prices in the developed world. Higher even than the comically inept regulated monopoly Telefonica. Which btw has phone plans and per minute charges a fraction of the US and where the idea for charging for an incoming call is unthinkable. And a food supply monopolies that inflates the price of food to stunning levels. The list is endless but my typing patience is not. So here's a link.
http://www.economist.com/news/briefing/21695385-profits-are-...
Ummmmm, no.
Whatever your economic viewpoint, there's no credible way you can claim that the Bell monopoly was healthy for the economy. This is the same organization that had to be taken to court to allow new devices to be plugged in. Remember these? https://en.wikipedia.org/wiki/Acoustic_coupler They weren't built because it's more efficient for computers to communicate over soundwaves.
The Bell monopoly invented the transistor. I think there's a credible claim that could be made that the invention of the transistor was healthy for the economy.