I think the reason a lot of leftists struggle with disability justice is that they haven't moved past the concept that discrimination isn't bad because it's objectively "wrong." yes, sexists are objectively wrong when they try to claim women are dumber than men. yes, antisemites are objectively wrong that jewish people are inherently greedy and run the state. yes, racists are wrong when they try to claim that white people are the superior race. and so on.
but then with disabled people, there are a lot of objective truths to the discrimination we face. people with IDs/LDs do fall behind and struggle with certain concepts. physically disabled people are often weaker and less capable of performing demanding tasks than able bodied people. many of us with mental illnesses are more reckless and less responsible. a lot of us are dependent on others and do not contribute much "worth".
and guess what? disabled people still deserve a place in the world. disabled people still deserve the supports they need. because they are people, and that should be enough to support them and believe they deserve a place at the table.
if your only rebuttal against discrimination is its objective inaccuracies, you are meeting bigots where they are at. you are validating the very concept that if and when people are truly incapable of being equal to the majority, that means they are worth less. this causes some leftists to then try to deny the objective realities of disabled people and/or become ableist themselves.
your rallying behind marginalized groups should start and end with the fact that people are completely worthy of life and equity, because they are fellow human beings and that should, frankly, be enough.
Although you’re correct on the ‘objectivity’ thing, I think you’re conflating things here, because ‘racist’, ‘sexist’, etc. are broad terms. There’s racism that says ‘those people are vermin and should be eradicated or at the very least kept on a very tight leash’ because they’re bad (supposedly because they’re genetically predisposed to be criminals, or because they’re scheming women), or because they’re a net drain on society, and then there’s the more ‘benevolent’ kind that masquerades as accommodation: segregating schools, keeping women in the kitchen, etc.
The last part is the most insidious, because it limits people under the pretext of ‘helping’, so a focus on saying, ‘No, we don’t need your kind of “accomodation”,’ is vital. Let alone tackling claims that those people are going to cause you tangible harm, and you can’t say ‘be compassionate’ towards people who are ontologically wicked. So tackling these things on the facts is a must here.
The way this carries over to disability is a separate issue I don’t feel entirely qualified to talk about. I’m autistic, but I’ve always had the feeling that I need to learn to adjust myself to society rather than the other wary around and use my autism as an excuse, and I’ve only started getting much nuance in recent years, and even then it’s mostly kinda-sorta and somewhat hard to shake.
(Semi-related: will everyone reading this please start adding alt-text to your pictures? Please, consider those who physically cannot see them.)
"you can’t say ‘be compassionate’ towards people who are ontologically wicked."
That literally is what I'm saying though. There are people out there who exist that are more dangerous or destructive or irresponsible than the base population and they still deserve rights. Some mentally ill people are more prone to acting out and/or causing violence, it doesn't mean they should be considered less than the majority. People's rights shouldn't be circumstantial or based on how well they behave.
People who suffer from a potentially treatable affliction that make them lack real control over their own actions or understand of them and their ramifications are not the same as people who are inherently inclined to harm you in ways they control and understand. People who choose violence this way definitely are less and we rightfully deny them certain rights. And even those who harm others and can’t help it have some rights denied as well, because you need to keep people safe. The issue is with those who say, ‘Let’s act preemptively because we totally can.’