I don't see why they can't just keep prices the same and add a "mandatory 20% service charge" at the bottom.
At least when tipping is "optional" restaurant owners can pull the "Well you can tip whatever you want, including nothing" card, but if you require 20% then you're just adding a hidden fee.
If you went to walmart and there was a sign at the front of the store saying all prices would be increased by 20%, that wouldn't be acceptable. Why should it for restaurants? If you really want to give 20% of every bill to the wait staff, then increase the prices of everything by 20% and give it to them automatically.
The case of Walmart would seem to go against your point; they're much closer to the tip-free model. "You see this price? That's what you pay." (With the caveat that some items are taxed.)
If anything, modern/Western retail stores closely parallel the transition that tipless restaraunts are making: there used to be an ad-hoc, inefficient, confusing, anti-introvert system for vending goods. It was called "haggling". It was replaced by a higher-volume, fixed price system that provided greater assurance that customers weren't getting ripped off or subsidizing those who were better at the dance.
Lots of parallels to how tipping works.
Edit: A closer parallel would be if Walmart replaced pretax prices and "you just have to know that some of these are taxed at whatever-the-current-local-rate is" with tax-inclusive prices.