Interesting to see our varying perspectives on 'value'. I avoid discussing headphone purchases with people outside of the hobby for obvious reasons; we all have our own idea of what a sane limit is for any given product. I think most would raise an eyebrow at $400, let alone $4000.
However I sometimes talk about this with a relative who is into watches and guitars. Both are hobbies with similarly stratospheric price ranges that can't be rationalised simply by material usage, manufacturing techniques, etc.
A big factor is resale value. The right watch or guitar (or car, etc) will hold their value and may even increase in value over time. As such you can appreciate/enjoy them for a period of time and then scratch that next itch; eventually switching to something else with minimal loss. An initially high entry cost turns into a low total cost of ownership.
But do top-end headphones like this generally hold their value?
I had a quick look and couldn't find any used Ether 2 for sale in the UK. Perhaps that scarcity bodes well. I got a really good deal on mine new and would be surprised if they sold for much less now. Not that I'd want to.
I've enjoyed my HD800S for over six years, and going by a quick look at prices on ebay, they'd still fetch £900-1000 (assuming they're actually selling at these prices); meaning they'd have cost me £70/year for that period.
I bought my LCD2C used and it looks like I could now actually sell them for more than I paid a few years ago.
The Focal Clear Pro doesn't seem to have retained value quite as well; I think because Focal have devalued their range by bringing out so many models.
A concern would be that if/when this technology does trickle down into other DCA products (as we hope), that $4K value could drop considerably. Perhaps most buyers lining up for these just won't care about this.
I personally think it's too much money to sink into a single set, but I understand the potential value proposition.