Never depend on a megacorp shit job for stability.

The way out is unionizing and workers coming together to form a worker-owned co-op business with other good people, a very low turnover rate, and a civilized environment. They don't need a megacorp and can find markets themselves.

What if even unions no longer have leverage?

That's a circular argument. Unions work when people band together because owners only understand money and output.

I’m not making an argument, I’m offering a thought experiment: “what if workers are no longer needed at all, so that they have nothing to threaten to withhold?” It’s a bleak possibility.

Then you have masses of idle, desperate, poor, purposeless people who either find coping hobbies to fill their time while some inevitably revolt.

Unlike the fallacy of the "lump of labor fallacy", there is always a limited number of job vacancies and limited number of employees rather than infinite open jobs just waiting around to hire everyone. (The pie is always a countable, finite number but never static, it can only grow or shrink within the means of finite-sized, finite businesses affording finite workers; the fallacy is believing in an infinite, unbounded pie that just magically grows because people exist.) Demand will/is rapidly shrink(ing) in many fields for certain roles, and the remaining ones maybe paid less and be more competitive to the point of extreme desperation. There are too many people left without a means to achieve fulfilling or a comfortable life because a few have reordered society to give them far more than they need and have taken away their livelihoods; history shows this rarely ends well without constructive intervention.

Indeed