Then it may be a while before we see the next new freighter model, because Boeing is happy to sell FX new 77F at good prices and FX is happy to buy them.
Yet https://en.wikipedia.org/wiki/FedEx_Express#Fleet suggests there are 50 or so MD-11s that will need to be replaced
Those -11s aren't going anywhere for at least 15 years. Right now, FDX has something like 60 -11s either active or rejoining the fleet and they are in the fleet plan indefinitely.
If FDX buys 350s, they are buying them for growth and frankly I think FDX has 50 350s of growth in pipeline over the next 10 years. The MD-11, like the A300 is in the fleet precisely because they can't afford to operate their current volume, to their current ramps, without them.