probably not unless they're forced to. retail lost money for amazon until very recently -- aws was the only thing keeping amazon afloat for years and years
which, I think, is the OP's point - if they spin off AWS, when they need to negotiate with the union then amazon won't look nearly as profitable as Amazon plus AWS: "hey we lost a billion dollars last year, we can't afford this contract".
"put it past them"? Why wouldn't the right reference be "what portion of the company operations would be disrupted if the unionized labor went on strike?"
If the warehouse workers go on strike and the profitable part of the business continues unimpeded, it seems like the retail/FC part of the business is the one that's at risk.
Retail reinvests money into fcs so that the amazon moat grows bigger. Anyone wanting to challenge amazon for delivery has to spend tens/hundreds of billions and years to replicate the fc/delivery systems.
AWS doesn't grow fast enough so they have revenue
Turning a profit is a failure. The market loves the forever growth strategy.
Wouldn't put it past them.