But actually to your other point. If I'm interested in working as a waiter at some restaurant, but I'm uncomfortable with tips being an expected part of my wage, then why would I either pursue that job, or accept it under those guidelines? Couldn't I work somewhere else instead and get the same amount of money?
20 years ago, it was definitely 15% minimum, but somewhere between 15% and 20% was very common (mandatory large party tip was usually 18%).
Regardless, you are correct that the expected percentage has gone up over time, and that a non-tipper or low-tipper is hurting the wait staff when they don’t leave a standard tip.
So you pay 15% on 2/3 of your meals, 10% on 1/6, and 20% on 1/6. But the mean is also 15%.
Giving 20% to everybody is Lake Wobegon syndrome, where all the servers are above average. If you don't adjust your tip based on the quality of service, you remove the incentive to strive for above-average service. You end up with servers that sit down at your table when taking your order and chitchat with you, instead of just doing their job professionally while you can see or hear them, and bitching about it in private later.
I've seen some other places with a different kind of tablet that simply has boxes for different dollar amounts of no tip, but they are typically not places that even have servers. Why would I tip at a retail shop when all I did was grab something off a shelf and bring it to the counter?