I think this makes it an interesting decision for those looking at the 400mm DO II. You can add 3 pounds of weight, 6 inches of length, and take off $900 in price in order to get the extra 100mm. For some, that's a good trade.
If you're the kind of person sticking a 2x TC on the 400, you can now stick just the 1.4x TC on the 500 and have a full stop of brightness better, as well as a little better focal length, and quite possibly better image quality due to having fewer extra elements involved.
It would fly off the shelves at $3500, but there is a valid best-use case at $6k. Just not mine.