Network Engineer here as well - I support one of the world's largest enterprise organizations (although my area of 'expertise' is pretty far down the stack - somewhere between the internet and the WAN circuits) - I can see no personal use for a 400 GbE port on anything.

As far as my organization goes, there will always be a bottleneck somewhere else (speaking location to location across the WAN).

The only applications I could see this being useful would be maybe huge ISP interconnects or maybe some big players in the video streaming world. That doesn't mean that future-proofing is a bad thing - of course cost will probably be a very large concern in the case of 400GbE...I see a lot of 40 gig ports that go completely unused, personally.