Thanks for the concrete counterexample. I still find it dubious that the owner of Bank A would be willing to destroy a stream of income as considerable as a 51% stake in BTC just to preempt a competitor in another market. That seems too self-sacrificial to be practical.
Also, you're missing the point that Bank B's successful adoption of BTC would also benefit Bank A's owner, since that adoption would be a boon for BTC, which Bank A owner has a very large stake in.