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I suppose that’s true.

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.

It’s been a while since I took a monetary theory class, but if people are interested in your first point they should look into fractional-reserve banking.

Essentially banks are only required to have a fraction of their deposit liabilities as liquid assets and can loan out the rest. If I remember correctly, this is actually how a substantial amount of money is created in the US and likely most of the world. If you deposit $100, a bank could make a $1000 loan assuming the reserve rate is 10%, which leads to an increase in the money supply of $900.

Money supply is definitely not the same as the size of the economy, but it is incorrect to say that bank deposits are just sitting stagnant. Banks are quite active with those deposits - how else would bankers make all that money!