Have you noticed:
Progression:
LBRS/Scholomance/Stratholme/UBRS -> HFC/Auchindon/CFR/CoT/TK
T1 Molten Core -> T4 Karazhan/Gruul’s/Magtheridon
T2 Blackwing Lair -> T5 Serpentshrine Cavern/The Eye
T3 Naxxramas -> T6 Mt. Hyjal/Black Temple
And as sideshows:
T1+ Zul’Gurub -> T4+ Zul’Aman
T2+ Ahn’Qiraj -> T5+ Sunwell Plateau
Do you remember how much effort Blizzard put into Silithus and Ahn’Qiraj? And that lasted all of what? A few months? Does anyone even remember going there? Do half of you even know where it is or what the story was? What was the loot like? Garish, colorful, odd. What’s the loot like in Sunwell Plateau? From what I’ve seen at mmo.champion it’ll be garish, colorful, and odd.
“The bugs are coming. THE BUGS ARE COMING!! Oh, look, Outlands. Nevermind. I’m sooo out of here. Race you to 70.”
Who goes to Zul’Gurub unless it’s on some lark? You can get better level 62 greens real cheap on the Auction House that run rings around the epic drops in ZG.
Naxxramas. I’ve heard they’re just going to recycle it like some summer-time rerun. That’ll fly to a discerning Blizzard player, eh?
As soon as they offered 61-70, all the 60 content was punted like diseased gnomes and players raced to 70.
As soon as they offer 71-80, all the 70 content will be punted like diseased gnomes and players will race to 80. They will race to 80, race to max crafting levels, race to maximize gear/equipment/epeen. And two weeks later wonder to the rest of the world “Is that all there is? I’m so disappointed with Blizzard.”
Last expansion they offered a two part expansion. 1) Race to 70 in a new world, and 2) explore four brand-new zones in the old world, with a new race for each, from 1-20.
Is the next expansion going to offer similar two parts? 1) Race to 80 in new zones in the old world, and 2) some new PvP action? Siege engines, an entire PvP zone set in every world? (I don’t think 95% of the playerbase exploring Gnome Deathnights with pink pony tails should count as a “part”.)
And it’s just a shame about the Sunwell Plateau. Requiescat in pace. We hardly knew thee.
Kinless Chronicles 
A nice theory, except.. Sunwell Plateau starts beyond Illidan / Archimonde. Archimonde and Illidan drop itemlevel 151 gear, the first boss in Sunwell drops itemlevel 154 gear. And according to some rumors, Brutallus requires around 2k sustained dps for each DPSer. That suggests that you need to have farmed Black Temple and Hyjal for a while to even have a fighting chance.
Sunwell Plateau might begin at an advanced Ill/Arch level, but how long will it last? By that I mean, will level 75’s be going there for gear? Since I suspect they’ll be racing to 80, more importantly, will the 80’s be going there for gear? (If you can raid BT/MH you aren’t going to pause to enjoy the scenery when the expansion comes out. You will power your way through till you’re stopped dead in your tracks by the content cap of the next end-game.)
Why do I care? Well, I think it’s a lot of effort and resources that goes into something quickly made obsolete. They do little incremental adjustments for over a year, then offer up a huge patch with some spiff-howdy content, and shortly thereafter release the next generation, making everything released prior to that irrelevant. They did it last time to the 60 stuff. They seem to be preparing to do the same to the 70 stuff.
Irrelevant might be too strong a word. At least turning a frontline place of action into a tourist attraction. Like the wife and I two-manning Live Stratholme. She’d never seen it. And then checking out some of the Dead side as well. Absolutely nothing in there for us outside of some boe patterns and gear for alts, and a chance to see the lore.
Content will always be consumed faster than it’s created, unless we’re talking about mindless grind content (like AQ resource gathering). I myself have lamented the fate of obsolete content, but not anymore. I realized this when my guildmate started organizing PuG raids into old content. I could have signed up and see C’thun for the first time. But I didn’t. Why? There’s simply not enough challenge. Level cap has nothing to do with it. I have stopped going to Ragefire Chasm. I’ve stopped going to Molten Core. I’ve stopped going to Shattered Halls. I’ve stopped going to Karazhan or the Eye. And that’s fine with me.
I don’t play games to be bored, I can do that without any games. I play because I like to be challenged. To me, the ideal playing session is where I’m required to adapt to a new situation, perform at my peak capacity and barely, just barely, prevail.
So yes, I’ll power my way through Northrend until I reach the content cap. And I’ll do the same to Nazjatar. Or Argus. Or whatever Blizzard thinks of next.