If they just store traffic and can decrypt it afterwards with the private key that they likely already protect with very high security, I think that's better than having to log traffic
and all the session keys which need to be protected in the same manner as the private key...
I agree that his message was a rational argument. The dismissive response was disappointing, however.
Ultimately, the conflict of interest here is created by the fact that not everyone wants the same set of security properties. (Rhetorical question: does everyone want DRM? It's "more secure"!) The important thing to keep in mind whenever reading about security is what it secures against. It is not unreasonable that banks want to audit the data that flows through their network, in the same way that I should also have a right to inspect the data my devices are communicating on my network. The example I like to remember is the smart TV that phones home with viewing info --- discovered because it was doing it in plaintext.
Rationale: We're trying to build a more secure internet.
Phrases like this rather bother me because it assumes everyone wants the same security properties, which is clearly not the case given this example. "more secure" against what? "secure everything against everything else" seems like a popular attitude recently, and I think it does more harm than good.
Relatedly, it's already hard enough with many "smart devices" that aren't full PCs to add your own CA and do MITM; I can't imagine logging session keys to be any easier if even feasible.
I'm with the banks on this one.