Possible ways, the banking industry can enter into the crypto field:
1. Provide buy/sell/swap service - which is very much possible and it can bring immediate customers to them but they may need different licensing to do the exchanghe service but it will be worth it.
2. Payment processing - (it comes under the swap service, though), there's a potential to fill in, if they manage to bridge the crypto buyers and fiat vendors.
These ways are just the best way banks can get into crypto without having to feel threatened by the existence of crypto because this way gets to benefit them so they just need to get involved and have another streem of income for the bank rather than trying to fight a decentralised system where almost no one has got a good level of control over. Payments processing is one are of crypto that will also help the banking sector make good money from the crypto market, I see that even exchanges are not offering this kind of services completely so if the banks can stand in, they become an aid that even retailers in the open market will be willing to explore and provide the bank with that high return in the end. The licensing will not.be a challenge for these banks to get because they have what it takes and it's very possible there's a provision for such in most regions.
These bank could have on their own accord taking advantage of crypto in this process that you have elucidated which can be of profit for them too with all the processing fees charges that will go along with. But the problem they face or are facing is that they are under the control of the Central Bank of the country they are operating, until the country's central bank gives a go ahead for banks that are willing to involve in the crypto market to get in, none of these commercial banks will want to go against the Central Bank policies that regulate their operations and scope.