I don't see this disagreement between you and your friend as a sign that your friend is not your friend.
It seems to me that disagreements on topics don't necessarily mean such.
Friend does not mean agreeing on all topics.
Your friend has a different opinion on this topic from you and most people here.
Perhaps he does not understand much about mental disabilities, which leads him to have his opinion.
Perhaps he has encountered people who do use mental disability as excuse, I am sure there are such people in the world.
Mental disabilities are highly varied and whether they can be overcome or moderated is variable too.
Neurodegenerative diseases probably cannot be overcome through efforts of the person who has them, as neurons are dying, and circuits are breaking down.
Mental retardation seems to set certain limits that cannot be overcome.
Something like autism is more complex and ambiguous.
Taking one aspect of autism like social disability, I would say that deficits in spontaneous social cognition cannot be overcome, but can be moderated if the person is able to learn some skills explicitly and practice them in the world.
To answer the question of the thread, I definitely don't have physical disability envy and don't want to be physically disabled in any way.
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Drain and plane and grain and blain your brain, and then again,
Propane and butane out of the gas main, your blain shall sustain!