I understand the frustration of traveling, and finding that the accommodations you are accustomed to and/or expecting/hoping/praying for are simply not there. I think this would be especially true if you were traveling overseas; I know that I always feel more vulnerable and wary when I travel someplace new.
In the US, the ADA (coupled with our HIPPA regulations) gives us a great deal of protection and freedom; I don't have to explain to anyone why I need to use the HA toilet, ramps, buses or hotel rooms, and although I have to have a signed letter from my doctor to get my HA parking permit, it doesn't say *why* I need one; only that I will benefit from it. Any system that requires a disabled or differently-abled person to "register" to use it will almost assuredly fall outside the boundaries of the ADA and/or HIPPA - so it won't happen in the US.
I agree with others here who have said that a Hoyer lift (or another other assistive device) would likely be broken and out-of-service in short order in a WDW companion/family bathroom. If you notice when you are at a Disney Resort, often the poolside lifts are covered by a canvas cover that helps keep curious kids and adults-who-should-know-better from playing with them.
In a perfect world, EVERY bathroom at WDW would be the same size, and equipped the same, so that there was no issue with the perceptions of the able-bodied vs. the disabled. I myself have experienced able-bodied groups and families using HA facilities at WDW; once at HS, I was waiting for the only HA stall to open, and when it did, a group of cheerleaders rushed past me and into the stall, giggling and laughing that they "beat her to it". Another time, at AK, a mother drug her three small children out of the stall, stuck her tongue out at me, and said "made 'ya wait, didn't I b*tch?" On two different occasions I have heard mothers outside the stall instruct their children to look under the door, and tell me to hurry up because they need it worse than I do. Everyone here in this forum likely has at least one similar story from WDW. Here's what's important to remember about those stories: The people who view those stalls or standalone restrooms as "prime" real estate, to be fought over, and to be hoarded and used because "they deserve it too" have no idea why those accommodations have to be so large and spacious.
They view it that Disney made those just for them - to use with their giant strollers, and/or their large families or groups. Because after all, isn't *everything* at Disney just for them? It's a logical extension of the same behavior we see when these same people walk right into the path of wheelchairs and
ECVs, or drop trash in the empty seat of your rental
scooter, or lean on the handgrips of a strangers wheelchair.
I'm not trying to start an "us vs. them" mentality - far from it. But we do need to recognize (just as we acknowledge and plan for the "zombie walkers" who dart into our path) the extension of that same mentality when it comes to *any* accommodation at WDW. If it can somehow be perceived as "better" by the able-bodied, then they will want to have that same "benefit" too - or they will simply overrun it. (see also: Family that steps in front of wheelchair user trying to watch HEA at MK in a designated area)
There are many reasons why we will likely never see twin bed size changing tables in companion/family restrooms that could accommodate adults at WDW outside of First Aid. Without being too gross, one would be adults using it for "adult activities". Additionally, I think there would be a certain percentage of the population that would go in there, lock the door, turn off the lights and take a nap - keeping the bathroom occupied for (potentially) hours (sadly, I know people who would do exactly this - and see nothing wrong with it). There would be families that would let their kids play all over it (think about where those kids shoes have been walking) while everyone in the family used the bathroom. Anyone who didn't realize it was meant to be an adult-sized changing table would just see it as a handy large surface! Great! And I shudder to think what manner of germs and "ick" would have to be cleaned off that surface prior to using it for a loved one.
We can debate all day long how horribly some Americans would treat such accommodations (and, sadly, it's true, they would) and we can talk all night long about what we *want* to have in an ideal world. Realistically, I think that WDW has struck a good balance, offering what they reasonably can, without causing the family/companion bathrooms to become a "nesting" spot for the first group lucky enough to get in there every day.
At WDW, I think the best solution would be First Aid in the Parks. Outside of WDW, it does become more difficult; Orlando's MCO airport has some HA bathroom stalls with adult sized changing surfaces and a sink, but overall, those types of accommodations seem to be fairly rare. The reality is that we have to be able to plan for and deal with these types of situations on our own. I wish it were different, and I wish there was more thoughtfulness put into accommodations everywhere, for everyone.