The problem is these ISP corporations are monopolies, there will be no competition for them, because in a free market the biggest corporation always wins, because they use their money to buy out their competition or price them out of the market.
So when net neutrality ends, there will be no competition because monopolies dont allow competition. Instead they will charge users more for internet access and control what users are allowed to look at on the internet.
Twitter and Facebook are private corporations that already do this, they can block content or users they dont like. When Net Neutrality ends ISPs will be able to do the same thing, they can block websites they dont like from being viewed by their users.