Ridiculous! It's nowhere near being dated, and won't be for the next 15-20 years. In terms of engineering it's still the most accomplished dynamic driver out there when it comes to headphones, and possibly the best engineered driver when it comes to headphones, full stop, regardless of type. Anyway, nothing else that came out after the HD800's were released comes close to it in the dynamic driver world. The driver is actually limited by the enclosure design, and you could get various types of sound signatures out of it, even in closed, fully sealed headphones, as many people realized when using the HD800's driver in DIY projects.
Anyway, we've reached a point in driver quality where going further would bring little advantage. The future of high end headphones is in digital sound processing, compensating curves that are tailored to each specific person, and various effects that will compensate the shortcomings of headphones on a neurophysiological level and that will compensate for the imbalances in the human hearing (no person has the exact same "frequency response" in both ears, that leads to huge problems in imaging while using headphones, due to the driver being so close to the ear and the interaction with the outer ear as a result of that proximity). Only by doing that will we achieve sound from headphones that will truly mimic sounds from the real world or even speakers in a good room. It's just unfortunate that all the attempts so far in doing that were inadequate at best, reason being, most of those plug-ins and apps are made by sound engineers, who (no offense) have nowhere near enough understanding of human hearing on a neurophysiological level, and therefore can't achieve good results. When medical experts start infiltrating the audio industry and start working with electrical engineers, only then we will have real advances in this technology. I might do that exact thing, considering I have a mechanical engineering degree already, and in two years I'll have a medical degree, combining these two will lead to amazing results, I already have plenty of ideas and am working on some with very promising results. The problem is, all the equipment required is expensive, and designing and building my own is out of the question in the next 7 years at least, until I finish my specialization.