I guess I should explain what I wrote above as a reply to dagezou.
He mentioned the popularity of some of the older girls. Mariko, Sayaka, Yuko, Kojiharu...no one's gonna argue about them being among the most popular. Consider though: "Among", not "the most popular", as he put it. That would be subjective. Anyways,...
To me, popularity ≠ reason or guaranty to be in AKB. Heck, SDN can be popular too.
You got to admit that until now, SDN has never reached the popularity of the original AKB. But! what if you just graduate a whole chunk of girls into SDN; let's say every member that as of April 1st, 2012 is 21 years or older (13 girls). Not that I seriously think that's gonna happen. But still... You wouldn't have just one, but two stupidly successful groups.
SDN would very probably sell much more than now due to Mariko, Sayaka, Yuko and consorts having such a great name value. And AKB would be able to progress, filling up the empty spaces with younger members from the lower groups (to most members of these lower groups, those members would of course be the older ones).
It's not like Akimoto didn't do it before (Noro, Ohori, Urano, etc...). Granted, these girls might not have been as "high-profile" like the ones mentioned above, still I find it not the least bit hard to imagine that Aki-P is planning something just alone those lines for next year.
Also: SDN's farewell performance is, although they could have waited another year, perfectly timed. March / April is the time when the new school year starts and maybe it's just me, but I find that quite symbolic.