I think roads are the best (and in some ways the most literal) examples of what municipal broadband should be.
The government builds roads past my house but it only provides "dark asphalt" (aka dark fiber), it doesn't provide any of the services that could be provided by the highway.
The government then licenses "service providers" to provide services on the municipal roads -- taxes for trucks that deliver things to my house, taxis, or even access fees for me to drive a vehicle on those roads. I have to pay myself to utilize the services provided by the roads.
Municipal broadband should be the same way -- it should only be the transit network, anything else -- IP connectivity/Internet should require me to pay an internet provider who in turn has paid for whatever access they need to the municipal network the same way businesses pay fees (direct or indirect) to use the roads to deliver services.
Comcast could sell TV services or Internet services, although I would expect that some other ISP would offer an better product than Comcast and they would be a marginal player, which of course is their entire objection -- hey have a rent-seeking monopoly they want to maintain. If the pipe to your house was open to any service provider, it seems likely they would only get a minority of people who wanted traditional cable television.