Big companies may save money on salaries, which would have first-order effects on the job market — but you could say the same about cloud computing, or computers, or mechanical reapers. Now what about all the companies that previously couldn't afford to exist because someone with a good idea didn't have six+ figures of discretionary income to throw at it?

If AI proves useful enough to effect sustained savings for Big Tech to the degree that you're suggesting, then the flip side of that coin is that it's effectively ZIRP on steroids for startups. The successful businesses which come out of that will ultimately have human labor needs of their own. Job market continues to trend upwards, only with lower concentration and greater overall resulting economic value in the form of more products and services.

Or AI/robotics/etc. gets so good that eventually no one needs human labor, and every company trends toward just being a CEO + board + AI. In that world, unless we expect everyone and their grandma to become an entrepreneur, something to the effect of a UBI would be necessary to keep the world turning without major societal upheaval.

On the other hand, AI skeptics can plan to sit back and eat popcorn while they watch the bigcos suffer the consequences of their mistakes and yield ground to startups. Either way, AI will have been a democratizing force.