The bigger you get, the more enemies you will have. I am quite sure that if Bitcoin becomes much bigger than it is now, that banks will somehow look to incorporate it or either do everything to make it imposible to purchase Bitcoin via their bank.
Incorporating in the meaning of assimilating it is impossible imo.They can adopt or link to Bitcoin, but will never be able to control it.
Bitcoin is all what Banks doon't like.And Bitcoin isn't the only decentralized, open blockchain out there.
Even if they would be able to take over Bitcoin, another decentralized Coin would continue where Bitcoin has failed.
This idea and technology in the way Bitcoin works is out of the box and will never go back into that box.
Money has a history of a few thousand years and central banking will be compared to that not more than 100-120 years.