I think that when you add 3 more theme parks and park hopping into the mix, it complicates things a lot. If it was as easy as just using the same system that Shanghai uses, WDW would have had it sooner. I agree that pointing at France and China and saying that's the test for WDW ignores the much more complex environment in WDW.
It actually simplifies park hopping. You could never have fast passes in multiple parks at the same time. With the system in Paris/Shanghai you can only buy the fast passes when you are in the park. No reason they can't treat each park individually in this regard - ie. you can only buy fast passes for the park you are in at the time. There are going to be big changes and what I posted is my best guess purely for the purpose of discussion. They are almost definitely not going back to the old system... if they were they would have done so before the summer.