A simple argument is as follows.

Without inflation, i.e. fixed money supply, it would be impossible for a majority of loans to be repaid with interest, or give a positive return to investments. Note that this fact is independent of how well the borrower or business performs. (This is like a zero-sum game).

So, it would be irrational for investors or lenders to part with their deflationary money, as the net profit is always zero (or negative because of other costs).

I am very interested to know if there is a way around this problem.