The problem with conspiracy theories is always this: the involved parties need to keep it a secret. You write "the banks", which implies that it's not only two or three guys. It implies that the leaders of a whole industry came together in order to plan such a thing. But as we know from real life, it's difficult to keep a secret. Whether we want it or not; sooner or later the shit hits the fan. For example the VW emissions scandal and there it was only one company involved and not the whole car manufacturing industry.
And finally let's consider that it took a lot of time until Bitcoin became adopted widely. It was more or less pure hasard. It took far too long to take off to be a planned thing.
Yeah that true. For a planned thing it goes really long.
Also, we know that the banks are not really far from exploring the blockchain technology and if they would have invented it, they would know it better in my opinion.
Maybe in the future, we are going to know if Satoshi Nakomoto is the real inventor or not. ;)