I don't think it's impossible for closed headphones (closed back and baffle) to be good. There are several out there that are - LFF's T50RP mods, Mr Speakers T50RP, NAD Visio HP50, Focal Pro and Classic. I have heard, and indeed modded for myself, any number of closed vintage planars to sound really pretty good.
High end? I haven't heard the new LFF or the Alpha Prime yet... but in the cases I have heard, not high end, no.
There have been attempts, like the T5P and XC, and while I haven't personally heard the XC from what I hear on this thread it has its issues just like the T5P does.
I actually believe it is perfectly possible for completely closed headphones to be high end (I have some ideas I'm not going to share in case I ever get to develop them) but the issue is that a lot of manufacturers don't design every part of the headphone with acoustic concerns in mind.
They make a chassis they think will look good, serve its ergonomic and functional purpose, then they set about tweaking the driver arrangement and damping the back chamber of the chassis until it sounds as good as it can.
The T5P chassis is a T1 with the back closed and damped. The XC is an X with a bit of wood screwed to the back and damped.
When I heard the headset for the EL8 was designed by a third party I did have a little pang of worry. Were they given a strict set of parameters which had been arrived at for acoustic reasons and then asked to add some aesthetic flourish? Or were they asked to design something that looked cool and then worry about damping them as well as they can later? I hope not the latter, because that is never going to get ideal results.
But I do believe it is possible as I said, and R&D needs to be poured into it as that is surely the end game of headphones.
Sennheiser invented open backed headphones to solve a problem, which was cool. But I feel since the advent of open headphones the industry has stopped putting any effort into solving that problem, seeing an open back as a permanent solution. It's not a perfect solution though. Sound leaks out, sound leaks in. They are essentially like speakers but a bit quieter; not suitable for listening with others in the room who don't want to hear as well, impossible to take outside the house into a busy area.
The true potential of headphones is a set that sounds as good as an Orpheus/009/pick your favourite, can be powered by a mobile device, and has perfect isolation from the outside world. We aren't going to get there without tackling the issues seriously.
) and was dissatisfied with every dynamic I tried afterwards.. Till I bought a HD700.. (Which, funny enough, is a polarising phone in itself) .. I then proceeded to read a review on Headphonia, which liked the tonality to the Audeze... Then it made sense!! As a footnote, everyone keeps going on about a treble spike, I can't hear it... 
