Unions don't end up with outright control over business. Generally speaking, companies and unions do continue to negotiate throughout the term of a contract, which negotiations are formalized in Memoranda of Understanding. And during contract negotiations, if negotiations over a particular issue stall, the law empowers companies to make a "last and final" offer, which is then included in the contract presented to the workforce. (This is U.S.-specific.)

The idea that a company must "agree to every demand" from a union seems to me a strawman. Bargaining ideally maximizes net value, and true collapses in bargaining are rare.

"Outrageous" demands that are met, therefore, represent a company's unwillingness to reallocate the value of those demands towards other contract elements (that is, to say "we can't do that, but we'll offer this instead"). If they were truly too costly, the company would have either a) refused the terms, or b) failed to bear the cost and gone under.

Yes, that's why amazon is hiring people who are expert in negotiations.

I think the problem is mainly the methods. Delta, for example, actively runs campaigns to deter their employees for joining the union. I don't know how is that perceived in general but I assume that people in general consider that more acceptable than the idea of running internal counterintelligence to prevent the formation of unions.

The reality is that Amazon is under so much scrutiny that pretty much any program on this area would be criticized.