I once left a 20% tip on a meal and the waitress literally followed me out of the restaurant. She shoved the tip toward me and said "if you're not going to tip well, then don't tip at all."
Since then, I rarely tip above 15%. I hear people say things like "if you can't afford to tip 18%, then you can't afford to go out to eat." Well, which would you prefer, a small tip or no tip at all? If you don't serve a customer, then you don't get a tip. So if everyone who tips less than 18% stops going out to eat, then you're going to have fewer customers, and therefore fewer tips.
Lastly, no one has to work as a server at a restaurant. I remember when I worked as a teller and a woman came in to make a $100 deposit. She told me that $100 was all she earned that week as a server at a nearby restaurant, and then she told me to tip my servers well. I didn't say anything to her, but I know she doesn't have to work at that restaurant, and she doesn't need to work at a restaurant altogether. There are grocery stores, warehouses, retail, fast food, data entry, telephone customer service jobs nearby. Those telephone customer service jobs start at $13/hour with no experience.
So regardless of whether I tip "below industry standards," I tip an amount commensurate to the level of service I receive. If my drinks aren't getting refilled or I don't see my server for extended periods of time, then the server isn't going to get a great tip. If the server is friendly, fast, and provides great service, then I leave a tip commensurate to their level of service, usually in the vicinity of 15%-20%. Sometimes (rarely) higher.
Having one stupid server doesnt excuse you lowering your tips overall. Also, I always appreciate twenty percent.
Telling someone they dont have to work there is ignorant. Noone has to work ANYWHERE. Next time you want to go out to eat lets see the manager tell you "Oh, noone works here, go fuck yourself, sorry".
Look, servers can have the money or not. Get greedy and you lose it it all. No one will tell me how much gratuity to give. It's not required, believe it or not. Servers would be wise to not bite the hands that feed them.
And, no manager would ever do that. Why? Because they want my money. Economics much?