I think MMJ could easily headline as well. Last summer 101X in Austin played the shit out of "Holdin' on to Black Metal" and they headlined ACL the same night as Stevie Wonder and a shit ton of people left Stevie's lame set to go to MMJ and by One Big Holiday, they had a huge crowd. I don't think a huge mainstream appeal has much to do with anything within festivals. Arcade Fire headlined and had a huge crowd at Lolla 2010, bigger than Soundgarden who were the opposing headliner, and AF were hardly a mainstream band, and didn't get "that" way until they won a fucking Grammy and suddenly they were everywhere to the point where their set at Coachella was the best show I'd seen and their shows at Bonnaroo and ACL left a little more to be desired.
MMJ have broad appeal, hipsters love them, jam heads love them, and they're unclassifiable. But they were selling out nice sized theaters for Circuital, they've sold out MSG twice (including NYE of '08), they may not be that popular, but I think they can become an even bigger band then they are now and I don't think their popularity is going to stay the same or decrease, they've increased their fanbase with each release and it's not like these guys are over the hill, they're in their prime, continue to branch out and with a festival that's always looking for new headliners and to give a band that big push, headlining Coachella on their next album cycle almost seems like a no brainer. Plus I wouldn't worry about them selling tickets to both weekends, after the quick sell out they had this year, I don't think GV will have to worry about sell outs for a while, especially when Coachella is "the festival" in the states.
As for TBK, I didn't think they'd headline this soon after last year, but it makes sense now, I probably thought they wouldn't have been able to headline back in '09 yet alone become one of the biggest bands on the planet. After Lollapalooza 2010, I knew they'd be on to something and could possibly do it, and here we are.