Is it possible that these companies simply fork the existing protocols and use the technology without buying into the existing "crypto world"?

Yes it won't be quite so decentralised, but say a number of major banks all spin up a node for say a JPM asset trading blockchain, it becomes semi-decentralised, so they have some advantage of a using a more secure shared ledger, but they also retain more control and thus probably more acceptance within banking, as big players can keep a walled-garden of sorts.