Level of service doesn't depend on the tip in the USA. Tip size may depend a little bit on level of service, but not the other way around. Basically the tip is how the customer pays the server and table clearers etc, because the restaurant owner does not. It's just a convention on how to split up the total charge that American culture has determined in the way to do it. And like every single weird thing in American culture, it's that way because of slavery.

Yeah but that's for restaurants. In restaurants, the tip is usually post-paid. Tipping in other industries may have a different dynamic.

For example, with Doordash and Uber Eats, tips are prepaid. The driver may also be made aware of their prospective tip during the bidding process. Therefore, tipping is gamified into enticing a driver into accepting the bid over other jobs.

I will not reduce a post-paid tip if I perceive that I got bad service. I usually won't increase it, either, without extenuating circumstances, for example, if the server overcame adversity somehow (especially if I was the adverse one). However, if a prepaid tip turns out to be unwarranted by horrendous service, I will go after that and require it to be reduced to $0. That is basically the only punishment that a customer can exact on a service worker. I don't like to do it, but if it was a prepaid thing, I consider that tip to be made under duress, and revocable.