MY Guess... LFR. Why bother to try and stick to a strict raid schedule for 25M when you can randomly jump into LFR where you + 24 misc "raid" and see the content? Just makes more sense since people are having busier lives and may not have the same amount of time they had during TBC and Wrath to get geared and do 25M content in reg mode.
LFR is more popular than ever just judging by the amount of complaints about the loot drops and gold bags. I would imagine LFR is the way raiding will continue to move until 25M heroic for guilds goes the way of the wagon in an era of jet planes.