At this point, I'm of the mind that even if there was one banksy originally, there's now several people behind the name.
If I was doing something like that, then reached the degree of interest that banksy did, I'd want to make it so that there would always be some small degree of doubt. The easiest way to do that is to work with other people, carefully selected, and make the work what matters by way of spreading the creation and execution around.
After that, even if I got unmasked, the work would still keep going, making said unmasking both pointless and dubious.
Hell, if I could manage to get enough people on board that could be trusted, it could still be me making the designs. It's almost all stencils, so there's never any guarantee that the person or people applying them made the stencils at all.
But the real point is that it doesn't matter unless you're one of the idiots trying to make money off of selling something done by banksy.