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.
Over expanding to fullfil a single large customer who isn't actually reliant on you is a good way to end up bankrupt. Seems like FedEx made the right move here.