The reason why they are used in studios is because there is something they do well, and that is provide decent value for the buck. They are not the best alternative, but rather a more economic one when you need multiple pairs such as is common in studio use. They are also used a lot by professionals when they are specifically needing to focus on speech/vocals because of the very pronounced leanings towards high frequencies.
As far as research goes listening to the majority and being swayed by the fact that they really are used by professionals leads people away from possibly finding better alternatves for themselves. After all like you said they were designed 30 or whatever years ago, they became almost a tradition, but a lot of time has passed since and better HP's have been put on the market in a similar price range. Especially HP's that are more useful and accurate in the studio environment as well as having handy features like removeable cables.
There is a reason why certain HPs do use coiled cables, instead of fixed ones.
If you need to move around, you are likely to be welcoming a cable which gives you freedom, and at the same time does not unroll into 10ft wire laying around.
If you listening anchored into a chair, a shorter, fixed length, cable can be less bulky. But for people like myself who moves around (couch, bed, or chair that is), the coiled cable saved both the amp input, and the HP cable inlet, a few times, which would have been pulled hard w/out a stretchy cable.
Granted, going on the subway with an iPod and the 7506 cable is likely not a pleasant experience, because the cable is bulky WRT 3ft of straight cable.
About the sound, as I tend to repeat in every post I make, subjectivity is rule. Or, "de gustibus non est disputandum". Hence the "listen by yourself" suggestion I give every single time.
But it is when you mention that sound professionals, which are people strapped to an HP 8h/day, do not care of finding better alternatives, either you have an hidden agenda, or you have met the wrong professionals.
Well there you go again attempting to make it a personal issue about me, something faulty with me as a person, or my thinking, motives or experience because I don't think the 7506 is beyond criticism. You are the one with the agenda and some silly need to try to discredit me as a person. It's like you are fueled by some extroadinary personal attachment to this particular HP and it's history and feel the need to defend it. Maybe it makes you feel superior to feel like you are attached to that history through ownership and me criticising them somehow makes you feel less superior. In turn you have to put some personal fault into me as a person in order to protect this shallow attachment to the storied legendary mythical perfection that is the great 7506. Haha get over it. It's just a $100 range product, and not a necessity. It's just a HP dude!
I was using them for studio use and know perfectly well what the purpose of a coiled cable is.
The coiled cable isn't perfect. What happens when it gets stretched is it doesn't have a strong enough memory in the materials to go back to the nice coils. When I used them I used them stationary at a desk. I wasn't moving around all over the place and stretching the cord out beyond what is normal for use at a desk, but nonetheless the coiled cable would find it's way getting bent out of shape and tangling up to the point where it did what I descriibed above. For your information a lot of professionals and everyday people had the same issue.
Further more you act like you know so many professionals. If you really did you would know these facts: professionals do not use the same gear. The 7506 is NOT used by every professional. A lot of professionals who started out using the 7506 no longer use them. There are whole generations of professionals that never used them. There are plenty of professionals that do not regard them as worthy enough for studio use and do not recommend them at all. Well, maybe for session musicians and vocalists to wear during recording and tracking, but not for mixing at the desk. Most professionals have multiple brands and models of headphones that they use for differing purposes and to get a different perspective on the sound meaning they are often not relying as heavily on the 7506 as you imagine. Also most professionals rely on speakers in treated rooms, not headphones, for the real accuracy of their work.
As far as their durability is concerned, well the cable is it's achilles heel. It's true that there are replaceable parts for it, and I do know that professionals are often replacing those parts or doing mods to make them more durable. I don't find the 7506 any more durable than most HP's on the market in actual real life practice. In fact I found them to be the least durable pair of studio cans I have owned or used.
Actually, go quick and make a thread on Gearsluts and find out exactly how many professionals would recommend these as the best studio headphone. Also ask them if they ever had or heard of complaints about the 7506. Ask them if there are better alternatives. Will you get a couple recommendations for the 7506? Yeah, maybe a few from bedroom producers and semi professionals for the most part, but it will never be a community or even thread wide resounding recommendation for them from professionals at every level of the game.
AFAIK gearslutz is the biggest and most easily accessible way to communicate with actual real audio professionals where you could read the liner notes on an album and be able to find some of those people posting and interacting.