Any breed can be a service dog. So seeing a Great Dane on Main Street tells you nothing.All I know is last year I saw a Great Dane walking through mainstreet in Dinseyland. I also know many people claim their dog is a "service" or emotional pet so that they can bring it into a hotel that does not allow dogs, or to avoid paying the hotel pet fee. It's awful but these people are out there, I called a lady out last year and she said I have the vest, i laughed and said give me a day and 60 bucks and I can order on from amazon too.
People abuse the system, absolutely. That doesn't mean we need to automatically assume that every dog that doesn't fit the traditional service dog model is a fake. As Gabe1 said, there are service dogs for PTSD, Autism, and more. Seizure alert, diabetes alert- those are service dogs.
Emotional Support Animals are not Service Animals. They serve a different purpose and are only protected in housing and airlines. If someone is claiming an ESA to get into a Disney park, they should be turned away. I think the odds of you offending a person who legitimately has an ESA are slim to none, since they know what the regulations are. ESAs are easier to fake than Service Animals but it shouldn't be the issue it is since they are only allowed in two places.
Education of the general public would solve a lot of these issues. Giving people who you think are faking a hard time doesn't- especially when you are wrong.