I don’t see how any of that is unreasonable you have to do similar things to get accommodations for planes or sporting events. Is it less convenient than what Disney does now? Yes but unfortunately human beings suck.
I think the general reason for the rules as they are, is that people shouldn't have to jump through hoops in general to receive a necessary accommodation.
99% of the time, this is fine, because there's no advantage to it. If someone needs help getting an item from a tall shelf in a grocery store for example, they don't need a doctor's note but it's going to take them longer to shop because they need to wait on getting help. There's no advantage to lying about having a disability in these day to day situations.
Theme parks, are the rare exception to this rule. Disability supports have a benefit of shorter wait times and avoiding paying for Genie.
So what do you do? Requiring a doctor's note to weed out the cheaters sounds good in theory, until someone with a legitimate need cannot access necessary supports because they didn't bring their medical documentation with them.
Disney over the years actually did a good job of levelling the playing field. By changing front of the line access to a time based return system, DAS provided support without being disproportionally advantageous.
Disney could eliminate paid Fastpass, again making the systems more comparable. Seems unlikely. They could request documentation. This could be achieved by tying it to the ticket buying process. Just like you can't buy a ticket without including a date reservation, there could be two clear options to choose from - no accommodation necessary or a clear guideline that documentation is needed.
Generally, I'd like to see a system that give people with a disability some advantage. Someone with mobility issues, for example, will find it harder to rope drop or will take longer to get around. Everyday things like using the restroom will take longer. By all means, let those people spend less time in queues.
The challenge is doing that without running afoul of ADA laws and stopping cheaters from abusing the system.