I help to look after someone with developmental difficulties similar to what the gp describes, and while I don't want to project my knowledge of one person onto the other, I can tell you that my disabled friend is never going to make it any skilled trade.
Look all these 'you can make it other parts of the system, I know someone who had a disadvantage and went onto a great success' is nice, but it's also platitudinous. It's a way of saying 'I don't want to deal with your problems.' Yes, there are successful people who didn't finish school or had learning difficulties, but they often had other advantages or had the good luck to grow up in a time or region where there was greater economic opportunity and fewer barriers to entry.
Someone who can't handle very basic things like multiplication tables is structurally fucked, to put it bluntly. That's more than not being good at math or not being bookish, that's not being able to work out the relationship between the stuff in your grocery cart, the bill, and the amount of money left in your bank account. And not in a way that you can you screw up a few times and then get the hang of, but possibly permanently and unalterably barring major advances in our understanding of the brain, which I guarantee you will go to poor and disabled people last rather than first.
That level of disability falls far short of normal adult competence and raises serious questions about whether the individual will ever achieve economic independence. People who are not very competent or who are incapable of financial independence are systematically treated like shit in this society and generally forced into the humiliation of relying on charity, which is inherently unpredictable and unreliable and antithetical to any kind of long-term stability.
I apologize for putting this so bluntly as it will be painful to read for the gp, but we need to acknowledge the reality of these problems rather than wish them away with a fairy-tale about enterprise and a can-do attitude. Life outcomes are more than simply a matter of high or low expectations, and insisting that everything stems from the expectations one has of someone's potential is an implicit abdication of responsibility for dealing with tricky social problems through public policy.
All these examples of being a plumber, electrician, or similar skilled tradesperson - which are often trotted out on HN - are basically restatements of what the various posters would do if you told them they had to work some sort of manual job. How the hell is someone who can't do multiplication at age 15 and has trouble putting a sentence together going to succeed as a plumber or electrician? Would you trust your home's electrical wiring to someone who can't work out Ohm's law to figure out what sort of fuse you need, or trust your plumbing to someone who won't be able to remember what sort of pipe to use for what purpose? Of course you won't, you hire a tradesperson because you assume they're certified and competent to work with sometimes-dangerous tools and procedures.
A person like that described in the gp may have many qualities, but intellectual disabilities like that are going to bar them from any sort of safety-critical work that might pay well. The friend that I mentioned at the top of this post has the intellectual and emotional development of a 9 or 10 year old. She's already a legal adult and her capacities are unlikely to improve significantly. That doesn't mean the world is a closed book to her or that she will never be able to develop herself, but it does impose a pretty solid ceiling on her economic prospects because there ain't a whole lot of jobs for adult-sized 10 year olds with personality problems. She wants to be a videogame character artist and I give her help and encouragement with anything that might advance that goal, but we're talking about someone who can't handle a busy shift at a Taco Bell and spends several weeks of every year in a secure psychiatric unit. She's almost certainly not going to go on to enjoy the rewarding careers people are talking about here, but very likely to end up homeless or in jail.
These are problems that we need to start talking about instead of wishing away because there are a lot of people who have a lot of problems, nothing much to lose, and little motivation to keep trying in a society that treats them as dispensable or ignores their existence altogether.