Let's assume for the moment that you're right and there are some people who can work, but chose not to, and use disability as their reason to avoid work, and claim disability benefits.
We can either just give them money, and not care if they label themselves as disabled; or we can make them jump through expensive hoops with complex bureaucracy and end up just giving them the money anyway.
I live in a city and county where I see recent immigrants busting their ass for minimum wage or lesser dollars.
I also see a PhD in CS spending all his time online and playing the victim for some made up disabilities (maybe there is a mental component). I'd like to see a bureaucracy that moves individuals like this into a required work/live support system.
We have a downtown cleaning crew of individuals with Down Syndrome helping out; we have recent immigrants busting their asses in the field, etc. Oh and we also have able bodied, poor mannered, self described disabled white people that refuse to do anything. My favorite was someone I knew through surfing out on "disability" for his back, but he'd still pull into 8-10' waves and was doing ding repair as well.
Personally, I like the idea of a basic wage, but I also know there are people that milk the system and want stop gaps for that too.
Basic Wage means those people aren't "milking the system" - they'll keep doing what they're doing, but it'll cost you less than it costs you now (because you've removed all the expensive bureaucracy between them and the money).
And you're pretty judgemental about mental illness. 20 years ago there was considerable stigma from employers, and less protection in law. Even if he'd wanted to work he would have faced discrimination. That discrimination still exists to some extent, though there are some protections. But now he has learned helplessness. People judging him doesn't seem to be getting him back into work. A programme following the Sainsbury Centre for Mental Health model ("place then train" - get someone a job, then support them and the employer to keep that person in the job) would be better, but those aren't common and they're always over-subscribed.
We can either just give them money, and not care if they label themselves as disabled; or we can make them jump through expensive hoops with complex bureaucracy and end up just giving them the money anyway.