That’s exactly why they can’t profit from it. The tractors for knowledge workers comparison would be accurate if the capitalists were merely expecting to cut down their human workforce, but they aren’t, what capitalists are trying to do isn’t just reduce the workforce but eliminate it entirely, they want to do away with the artists, the researchers, the programmers, the accountants, the lawyers and even the very workers who are making these AIs for them in the first place.

But that cannot work because as you said value comes from the pairing of labor and technology, so what’s going to happen is that, at first, the first few companies to figure it out will, yes, increase their profits massively by taking advantage of their more efficient than average production process to propose a slightly lower price than competitors can offer, but only for a time.

As Marx showed, once a new productive technology spread through the industry competition and the price war comes back with a vengeance and forces the prices down to the level of the new value = the cost of materials and running the machines (dead capital) + the value created by workers using the machines to produce (the worker’s wage + the surplus that the capitalist takes).

In other words, if AI works out the way the bourgeoisie wants and they are able to eliminate labor completely, there won’t be any workers to create new value i.e. no surplus for the capitalists to pocket. They will be stuck selling at the break-even point (or at best so insignificantly above it that it doesn’t really matter) without being able to walk away because any who try to hire labor again will have their costs go up and therefore be out-competed by the fully AIs companies.

AI as the capitalists want to implement it makes profit making impossible. Sure, that doesn’t mean they will lose their wealth, as I said they will be forced to sell at the break-even price neither profiting nor losing so if they make sure to plug any wealth sink they might have they will be able to keep their fortune.

But the point is, that’s the end of the road for them, without the ability to continue accumulating profits there is nowhere for them to go, no direction in which to take their ventures, all they can do is sell everything and spend the rest of their lives off off the huge piles of cash they managed to make when business was still profitable.