If indeed this is the case, we must ask ourselves is what is correlation, and what is causation? Do guilds which progress get to a spot where they hate each other, then reform over and over which leads to congestion, or maybe congestion leads to a lack of targets and guilds begin to hate raiding and hate each other, then break up and reform and try to bash their way through that crowded tier all over again.
There are a few guilds that are based on a core group of friends that really like hanging with each other, and also happen to be raiding. They progress at whatever pace they can with their raid schedule, and it may be really slow or no progression one month, and next month every raid is a new target. They're just friends, and they hang out through raiding.
Then you have guilds that are there to get loot. Yeah, there might be some small relationships in them, a couple groups of friends, but there is no guild loyalty or desire to be there beyond getting the loot. You see these same 6-8 friend groups form a guild, recruit new people, get to the same slow point in progression their last guild got to, and the people there riding along for the loot (everyone that isnt those 6-8 people) start to fall off cause the loot and progression slowed down.
Then these 6-8 people reform again, and the reach their stagnation point in progression and their guild falls off.
That being said, I don't think getting rid of current congestion is a needed thing. Current congestion is player made, not because there isn't enough content. There is definitely tons of content to go around at this point.
The biggest problems I see in low-tier raiding is guild longevity, which the developers can do nothing about. The biggest problem with new High-tier raiding is that often it is too limiting (6 man content doesn't help guilds progress, it helps the "elite" 6 people in a guild progress, and maybe later once the content is trivial they can swap 1-2 of those elite out to gear up some of their guild mates, but at that point their regular 18 man raids are getting better gear anyway).
Emberflow and Cmal 4-2 are fun and all, but they don't help progression on an equal scale to say, the introduction of PoFrost did. They also lead to resentment between guild members. Oh, I totally love playing my enchanter, but until we've progressed to the point where Cmal 4-2 is basically not an upgrade anymore, nobody will even take me there because I can't contribute very much in the 6man setup. Its not a problem with enchanter who operate fine in a 18 man setup, its a problem with content being too limiting, with too narrow a scope. At that point you can't blame the players that want that awesome 6man loot and happened to enjoy playing one of the 6-8 awesomely useful grouping classes for doing the content, and excluding those other classes that just don't contribute as much. You have to look at the devs for making the content that by design excludes a significant part of a guilds raiding roster.