We'd get fine service.... as long as we're going to sites that the ISP hasn't throttled (so either sites that pay your ISP to allow more bandwidth to be dedicated to their site, or those that the ISP is trying to promote). Otherwise.... yeah, the service is probably gonna suck. And it's super easy to see why.

ISPs are companies. And companies really don't care about the consumer, as long as they make a profit, or there's actual federal/state laws that dictate that the company must operate according to certain guidelines and standards. Without Net Neutrality, said laws will no longer apply to the ISP, allowing them to do essentially whatever they want to how much bandwidth they provide, as well as the prices they charge for those services. Which basically means you'll have to pay way more money to get good internet service.

This is simple human nature at work. Humans have a history of screwing others over to get what they want. that's why laws exist, to keep people from utterly screwing each other over, or at least, the more powerful people from screwing the average joe over. And without Net Neutrality, the ISPs become the powerful people that screw the average joe over so they can get what they want: money and influence.

Which is extremely bad in the case of internet, given how much people, and businesses, rely on it in this day and age.

And boycotts won't work in the long run, as there's no point in ditching your shitty internet service for one that's equally shitty. And even if people boycott one ISP, and the others improve their quality to draw in the boycotters' business, as you say, the service will just go back to shit once they get your money.

Businesses don't care about the people, only themselves. That's why we can't act like businesses are people.