A family member of mine owns a restaurant that does mostly delivery business. He employs his own drivers; he's on GrubHub because it brings in more business and is a net positive for his bottom line. He includes flyers with every order that says, in large obvious text, "Call us directly at [this number] and save 20% compared to GrubHub."
He still gets a sizeable amount of repeat business from GrubHub.
I've experienced the opposite. I used to order pizza on a phone from a local shop a few times a month but then one day a machine answered the phone telling me to order using Grubhub. I've never bought anything from that business again.
He still gets a sizeable amount of repeat business from GrubHub.