For anyone who needs a (very simplified) primer on the blockchain and why it's relevant for banks: A blockchain is a ledger (piece of paper that records transactions) with perfect memory of every single transaction that has ever occurred within a certain network. This makes double spending within the network impossible as you can trace every single coin in the network back to its origin, verifying that your trading p…
If you have too many blockchains, then you split the miners between the chains and open the doors for a big player to come in and overwhelm the other miners on that chain.
I suppose you could somehow implement private blockchains where only those with the private key can participate, but I don't know whether this has been done or not.