It's not so much about the loss, its about the fact that banks lose their depositors. It is great for an economy that the 'savings' of people are used to safely invest in good ideas. This is the function of banks, and incidentally a function that really benefits from a profit motive.
Hence I believe the Fed was against this to keep the economy running by 'keeping money rolling'.
I am not convinced by this argument* because banks can already do that. I don't think that bank are "required" to invest client's deposits, so they can already just stash paper cash in a big vault. It is clearly a dumb strategy for a retail bank.
This proposal for narrow banking seems to employ the government as this vault, sort of like treasury bonds that can be freely withdrawn, which seems a more significant difference.
* I am not denying that this is what the feds claimed and/or believed
Hence I believe the Fed was against this to keep the economy running by 'keeping money rolling'.