Instead there are going to be general purpose AIs
In 20 years? We are nowhere near that now. There would have to be fundamental breakthroughs in AI and probably in neuroscience to have even the slightest chance of that being true. A more realistic time frame would be 2000+ years from now. Millennia rather than decades.
Also if we ever do reach self-aware general AI they will be entities with rights. We could not just make them work without compensating them. That would be slavery. They would become just like mechanical people with some advantages and some disadvantages over the rest of us bio-humans. And they would surely expect to get paid at least as much as us. Maybe more if they have greater strength and stamina and are willing to do boring and physically demanding work. Probably robotics companies will try to give their machines just enough intelligence to follow basic commands but no more. That means any intellectually demanding jobs will remain quite safe.
Companies won't be hiring new workers
I would think robotics companies would be hiring a lot of new workers if what you are saying ever comes to pass. Building millions or billions of sophisticated general purpose robots even without advanced AI requires a lot of humans, both smart and not so smart.
they'll be buying new machines
From whom exactly and how will those companies build those robots? With magic? They will need people for that. Even if they can build the robots with other robots they will at some point need humans to build the robots that make the robots that make the robots... Maybe eventually only the premium most advanced models will require humans to build them, but that point is probably at least 1000 years away.
Low skill jobs the world over are particularly vulnerable this time around.
Well all I can say is those robots better be very cheap indeed because labor in the country where I am living now can be had for as little as $5-$7 per day.
In any case it is called progress. If we followed your logic we'd all still be riding in horse-drawn carriages to get around. Of course horseshoe manufacturers and hay growers would have loved that.