"demand" is not only willingness, but also ability to pay. If a business is going to pay employees more, it has to come from somewhere. If rents are rising and investors have other places to put their money, then it's going to have to come from the customers. And the customers don't have infinite money any more than anyone else does.

There are plenty of services that provide value to society, but don't necessarily bring in massive amounts of revenue. There's even a value to the workers themselves, the pride in providing a service to customers on need rather than having to come up with sketchy ways to squeeze yet more money out of the upper middle class. And don't forget: restaurant owners aren't rich. Most of them aren't even upper middle. Divide their take-home by the number of hours they work, and many small business owners are doing poorly, sacrificing income for the sake of running their own business. The workers need to be able to sustain themselves, but just as most of the income of the rich comes from a source other than their labor, there's no reason 100% of anyone's income has to come from their labor.

A UBI would put more money in both the worker's pocket, and their customers. Not only would it make it possible for workers to sustain themselves without breaking small businesses operating at the margin, it would also make it easier for the customers to absorb higher wages, should it become necessary.