But then, the problem is that you have a cartel, not a duopoly. That’s the thing: you can only punish companies for their actions. A duopoly is a fact, in itself it does not imply any particular behaviour from either company. If there is collusion, then it’s anti-competitive behaviour, abuse of their dominant positions in the market, etc. Things that are already illegal and should be enforced.

The reason there is only 1 broadband ISP is because people are not willing to pay sufficiently more for fiber to offset the costs to install fiber to the home, especially in places with buried utilities.

Therefore, the existing coaxial connection is the only economically viable option.

Also, it rarely makes sense for 1 home to have multiple physical infrastructure connections, so they lend themselves to natural monopolies. If a house has access to fiber, it makes no sense to spend resources to run another fiber to the house.

Which is also why ISPs should be utilities, but that is not comparable to personal devices.

Which might be the case if, through taxes, we hadn't collectively paid for a lot of that in the way of subsidies and grants to those ISPs to do exactly that, subsidies and grants which resulted in, generally, more dividends, bonuses and stock buybacks than they did miles of fiber being laid.