I worked briefly at FedEx corporate, and they made a big deal about their policy that a customer could never account for more than 30% of total volume.
Leadership often made it clear that Amazon was right at that limit, and wanted to send a lot more volume, but FedEx wouldn't let them, in order to maintain "independence."
To your point: it didn’t stop them from working to cater to Amazon’s every whim, and it did provide Amazon the incentive to build a (better and more cost-effective) fulfillment and shipping network themselves.
I don’t know what the “right” play was, and obviously the story is far from over. But FedEx always seemed to me like they chose the worst of both worlds.