Thinking doesn't mean finding the theory of everything. It can be as simple as taking a walk and see what people who have money are currently doing that you could do for them.
Cleaning lawns, walking their dog (scales wonderfully, as you can walk with more than one dog at a time), buying their groceries for them (easily worth it, as you would only buy exactly what was on the list and not 'hey this is cheap') and it scales reasonably.
That list is by no means exhaustive, btw. Just of the top of my head.
None of these can be done by robots anytime soon, none of these require a high school education.
Yes I have some idea of how though this is -- my granddad started working as unskilled labor at age 14, and he raised both him and my grandmother from homeless near orphans to living in a (small) house with a car and in the process raising two kids to have families of their own.
Your grandfather, while not educated, likely had his wits about him.
Cleaning lawns often requires average intelligence. If I have to tell the person common sense things and basically supervise the work (people with this intelligence often don't adapt well to change, the kind of change required for each customer) and will tend to be slower and require tasks to be broken down for them. You don't want to have someone completely break down when you send them to the store and the brand you specify isn't available.
From people I've seen, they're already barely functioning in jobs. As the job market gets tighter, many of these people have been pushed out. Some have had advocates able to argue them on to disability, but others haven't.
You could do this. I could do this. But you're overestimating the capacity of 10% of Americans, perhaps as much as 15-20% of Americans.
In other words, if you expect your life to continue as it has always done, it won't.