I wish to clarify.
If I were waiting for a lift and a person in a wheelchair/other disability were also waiting then, when the lift arrived, I would invite them to enter it first (and have done so) because I figure it's easier for me to nip in as the door is closing than it is for them.
However, I have also stepped in first, if I am standing at the front, and pressed the 'door open' button to ensure the doors are open for long enough for them to get in.
This is because I am an adult and they are an adult. I will have evaluated the situation and taken what I consider to be the best course of action. Not because I think they are incapable of making that decision, but because someone needs to enter first and, to be honest, given that I'm not an animal, I would always invite the other waitee to enter first.
Sometimes the person in a wheelchair says "no, after you" (because they are also a reasonable adult) and sometimes they just say "thank you". Because they are a reasonable adult.
I don't ever presume that because someone is in a wheelchair or has another disability, that they are incapable of the kind of normal social interactions that I would expect of someone who wasn't in a wheelchair/didn't have an obvious physical disability. Neither do I presume that they expect me to engage in some odd little social dance where we determine a hierarchy, nor do I wish to patronise them.
If there is not enough room in the lift for both of us, I will also invite a wheelchair user to enter the lift instead of me, because I am polite like that. But I would also make the same offer to anyone by virtue of the fact that if there is not enough room for me and them, then I would always offer the lift to another person. They are then free to accept that offer or not.
However, if I am in a lift that opens at a floor and the person waiting is in a wheelchair and makes the decision that, due to the number of other people in the lift, there is not enough room for them, I would expect them to wait for the few minutes, like anyone else, for the empty lift on it's return journey.
It wouldn't occur to me that when the door closed the person was sitting there seething and thinking the other lift passengers were BU! I would assume they are thinking "Oh the lift is full, I'll get it next time". Because that's how it works.