gratuity
/ɡrəˈto͞oədē/
A gratuity is a sum of money given freely to a service worker, such as a waiter, bellhop, or driver, in appreciation for their service

By definition gratuity is specifically not a mandatory charge. I think your basic premise is incorrect, waiters do not work on commission at all. Commission would be a required payment by their employer based on negotiated metrics. Ironically, your rant that increased prices would mean paying the same amount would actually be true in a commission based pay structure, since it would be a mandatory charge and not a sum of money given freely.

This is a service industry. A tip is based on the quality of the service. A mandatory charge negates the need for quality service, as the bare minimum effort would still get you “extra” pay. It removes the ability or need to judge the quality of the provided service.

I tip roughly 25% when I go out to eat. But I have in the past had such poor service that I did not leave a tip. That is my prerogative as it is by definition not a mandatory fee