I do think that your original argument that bank deposits don’t contribute to economic growth is wrong though. As you point out, they are a cheap form of liquidity for lenders. I think you’d agree that loans play a key role in economic growth.
The ability of banks to lend is limited by the availability of credit worthy borrowers, not liquidity. Banks will always get the necessary liquidity from the central bank (assuming the system is working as intended), but prefer deposits because they are cheaper than whatever is offered by the CB. Inter-bank lending is an alternative preferred option if insufficient deposits are available.
I do think that your original argument that bank deposits don’t contribute to economic growth is wrong though. As you point out, they are a cheap form of liquidity for lenders. I think you’d agree that loans play a key role in economic growth.