Had this typed up then got busy so it timed out...
We are comparing the first and second raid tiers of MoP vs. the last tiers of Wrath and Cata; the last tiers both running a span of over 8 months, vs. tiers that will last 6 months tops. Also, consider both ICC and DS had the stacking raidwide buff over time. I daresay it helped to delude people's expectations of the difficulty of heroic enounters...but this thread isn't about heroic encounters.
Regarding ICC 10 man, before the stacking buff eroded the difficulty, you had 25 man guilds (at a time when there were as many or more 25 mans than 10s) who would divvy up members and run 1-2 10 man runs the same week, since the lockouts weren't shared. They would come in the door in what amounted to 10 man heroic gear, due to ilvl difference.
With DS, I would think the number drop off from guilds on Marrowgar vs. Morchok has more to do with what occured earlier in the expansion, with the first heroic dungeons + T11 being very punitive compared to the lulzfest we had gotten accustomed to in ICC, with it's 30% buff and a LOT of people being in mostly heroic 25 man gear.
Also, just because numbers are dropping off (this point applies more to ToT) doesn't mean these guilds are just vanishing. Of, say, the 5k that downed Jin'rokh but aren't account for on Horridon, that doesn't mean they aren't still logging on and trying to get him.
I dread to see what normal mode numbers for T14 and T15 would look like had LFR not been implemented.