Unions are probably the biggest threat to Amazon in the long term, as their interests are directly against Amazon, they'd rather there be 100 competitors they can buy out/drive out of business one at a time; then a single union who could easily shut them down (by striking)
It's easy, spend 100M dollars on a law that makes warehouse labor a mandatory union.
Mom and pop companies can't afford to fill out government/union paperwork for their 3 employees and customers can't go to cheaper alternatives.
But this means Amazon and Amazon employees win.