Legendary did break the game, and all you need to do is take a group of legendary into any pre-legendary zone that would have demanded at-least 8+ spelled/stacked chars and you'll realize that the same thing can be done with far, far less when legendary is used. So, legendary did break the game, in fact it pretty much smashed virtually every single zone. But there have been revamps and new zones added since legendary was added, and those changes took legendary into account (thus why things at the very end are even harder now).
But, even with the changes it's still very possible for any clan to do the end zones/fights, provided they have the drive and dedication and are able to build up the strength in the first place (ie. ranks/spells/eq). They also need knowledge, and I would argue that knowledge is the single most important factor since it's also the hardest one to achieve (unless you get it handed to you).
But, if you want the "root cause" then I see it as being fairly simple, and obvious. It takes a LOT of work, time, and dedication to build up the strength/knowledge/experience it takes to do the hardest area's of this mud, so you won't be seeing a group that got thrown together from whoever was still around having much of a chance, especially if there's a distinct lack of knowledge about the specific zone/fight. It takes an entire clan working together for weeks, or month's, to be able to do certain fights and zones, so without some actual direction and intent on being able to do the best of the best in terms of zones/fights... *shrugs*
