The Dolphin isn't exactly "offsite," though. Swan/Dolphin are physically located on Disney property, within walking distance of DHS and Epcot, they get pretty much all of the benefits of an on-property Deluxe resort (and they had Disney-run transportation pre-pandemic), and Disney does get a cut of their revenue.
That being said, Disney is still getting significantly less of your money, and I think you make a good point about people paying for Genie+ by cutting costs elsewhere.
Disney execs seem to be making the assumption that people are going to spend more for Genie+ on top of whatever they were already planning to spend. However, at least for us, that's not the case. Our trips have a set budget, so if we do choose to purchase Genie+ for our upcoming trip, we'll just reallocate the cost from somewhere else in the budget. Disney won't get more money from us by charging us for Genie+ because we'll just spend less on other things.