It would be very difficult for new companies to enter the market because people would not be able to call new companies, since they aren't under specific plans. The established players could effectively block new competition or make it incredibly expensive to enter the market.
- All car dealerships should sell all brands of car.
- All grocery stores should sell all foods.
- All music streaming services should stream all songs.
- All movie streaming services should stream all movies.
- All movie theatres should play all movies.
- All phone companies should let you call all numbers for free.
- All radio stations should play all songs.
- All TV channels should play all shows.
- All cable providers should offer all channels.
- All devices should support all operating systems.
- All airlines should provide flights to all airports
- All stores should sell the same items for the same price.
I have yet to find anyone who supports a majority of the above statements, or is able to explain how they differ from net neutrality.
Your questions don't get answered not because they're devastating to the pro-neutrality position but because they don't address at all what net-neutrality proponents actually believe or why.
The other major issue is competition. All the above examples have a lot of competition, but internet companies are a lot of times monopolies and duopolies. The consumer has no choice but to use them.
It's very common for carriers here to give unlimited calls to 5-10 people of your choice. Likewise, it's often cheaper to call people on the same network you are.
A lot of companies have 1-800 numbers you can call for free.