"but everybody I ask to comment on the idea tells me about Grubhub and similar services, and they just don't understand why there would be other kind of service."

Who is everyone?

Is "everyone" the target customer?

Are is "everyone" startup people, programmers, hackers, VC's etc?

The fact that there are already sushi restaurants doesn't prevent new ones from opening everyday and doing business and making money.

"The great thing about automatically building webapps for delivery restaurants is that this can work"

Exactly. Not everything needs to be a home run.

"probably because of the fees"

Right. And this is especially a big problem with the mentality of the small business owner. Not wanting to cannibalize existing customers.

Building the app and charging a fixed monthly fee is the way to go at least to start. And make it easy for the restaurant to make menu changes etc. In the beginning you would have to perhaps input all the items yourself because the small business won't want to take the time. And have to prototype at no charge to prove it actually has benefit.

Let me know if you end up doing this.