I don't think BJJ is the pillar of MMA any longer. It might be Wrestling for the ground. BJJ is def. still there, but the main reason you will always see BJJ in all MMA gyms is because that's what makes the most money for the gym owner. BJJ has an enormous circuit for competitions, so it gives hobbyists, medals to shoot for. You can probably find a BJJ competition event every month. The popularity is just huge compared to the little that there is for TMA.
There are good reasons why MMA fighters don't flock to Wing Chun, Aikido, etc. to train. Their styles are just inferior. There are some techniques that can be used from TMA, mostly for surprises, but they're not bread & butter techniques. Like Jon Jones took up TKD to get in some sneaky kicks and you can see some Capoeira in McGregor's movements....but they ain't taking up TKD & Capoeira, full time and lose millions.....in McGregor's case, it could be $100,000,000+.