If they grow to 20T then buying the index will also be a good investment but with a good risk hedge.
Not if open weight models turn inference into a commodity that's sold at cost.
There will be, as is always the case, some amount of new labour skills that emerge and I can't say for sure what percentage or amount that might be.
Looking at tech work being increasingly well the last 20-30y, we can imagine new ways of working might be pretty valued.
Most technologies seemed doom and gloom, and also have absorbed existing jobs for new ones, and current LLMs as AI is a bigger sponge than usual.