You've got it backwards. Cable and telephone companies were granted local monopolies all over America, which is why there is a legally enforced duopoly now all over the whole country.

Monopolies only exist when created by regulation.

By the way, to head off a common concern, by example: People who think Microsoft ever had a monopoly are using the word "monopoly" wrong. Consumers were always free to go a different way, or start a competitor (and many did). In fact, MS would probably be weaker today had regulation not forced it to improve its self-defeating business practices.

If you would like to bury your own wires and use them to deliver Internet, you can. It is just like any other significant construction effort where the costs & paperwork may be daunting to some.

I was presuming that when the company agreed to build a local network, it got an agreement from the local authority that nobody else would be granted a permit to do so.

That's still a legally enforced duopoly. Or are you saying, that acutally doesn't happen?

Cable franchises restricted competition for wireline video delivery but that got tossed a few years back.

I am guessing that's because the phone companies also wanted to deliver video over the wire. So this still wouldn't break the duopoly.

Incorrect.

A "de facto monopoly" is just "very successful in the market."

There is no need for a separate concept for this, and if there were, it would not be appropriate to overload with the pre-existing term "monopoly" that means something different.

If you take the dictionary definition rather than the legal one, sure. The legal one is the one that's actually relevant for most people's purposes, though.