Well the thing is it isn't rendering anything irrelevant. If people really like that content then they'll do it anyway. I've personally leveled a good... mmm... 40 characters from 1 to at least 30 in my time since Vanilla. Probably 20 of those characters made it to Outland, 10 made it to Northrend, 6 hit level 85 and 5 are at 90 (my poor paladin ;-. Do you know how sick I am of that old content? I'm the kinda guy who LOVES achievements but can't bring myself to get Loremaster because of how boring questing is for me at low levels.
The only issue I see is new players not understanding that the old content can be exciting the first time you play it through and quit after they get bored of level 100 stuff. If my little brother doesn't hit 90 before WoD comes out, I will most definitely make him level through the old stuff after he hits level 100 and possibly decides to quit due to lack of interest.