I guess 400mm DO was too cheap ,they want rather sell same weighting 500mm DO for 15k
I am not 100% sure, but I recall to some level of certainty that the DO and PF lens elements are hard or take a long time to make and that is why at least Nikon build 300s and 500s in alternating batches. Perhaps Canon has to free up the DO glass for new RF lenses?
Come to think of it, is this perhaps a reason for the 600 and 800 being f/11? Or even some attempt to scale up DO glass production for the big whites.