One of the unrealistic expectations across crypto, in my opinion, is that the leading permissionless blockchains (Ethereum, Cardano, etc) will manage to totalize the financial fabric of tomorrow.
They will almost undoubtedly play a profound role and, once we get more comfortable and familiar with the tech, enable a better degree of financial inclusion, but the incumbents financial institutions around the world are unsurprisingly choosing permissioned blockchains.
The difference? Permissioned networks only include nodes (computers) that have been approved, one way or another, by whichever party has authority over the blockchain. Makes perfect sense. Last I checked, Wells Fargo is going with Paxos' permissioned bespoke blockchain rather than, say, Ethereum, because that way they get to supervise the protocols and mechanisms that best suit their business model.
If the likes of Wells Fargo went with a permissionless blockchain, all it would take is an influx of nodes operated by those with intentions contrary to big banks (there seem to be a few here and there) and, generally speaking, new updates can be forced upon the blockchain by a renegade majority.
That said, there could well be means of nesting a permissioned blockchain within a permissionless one. Having some exposure to the likes of Ethereum, as a sort of blockchain commons, could reasonably be in the interest of this or that financial institution enterprise.
But I'm far from convinced this is going to flush out the major financial institutions. If anything, it could optimize their business models, from T-2 to T-0, provided they jump on it now, which they sure seem to be.