Yes, we agree up to this point.
You're saying that any venture which results in an increase in real value is a good investment, which ignores opportunity cost. An investment with a 900% increase in real value is a bad investment if there is an alternative which would generate a 1000% increase.
I agree that deflation has no effect on "inherent productivity", but these "moderately productive" investments are actually destructive, not productive, because resources expended on them take away from even more productive investments. Thanks to prior saving, there is a certain amount of surplus available to be expended on various investments. If you hold on to your money, that surplus is available to be used by existing ventures, with an expected return equal to the rate of deflation. If you invest your money in a venture with a lower real rate of return, that portion of the surplus is no longer available to be used by the ventures with higher returns. The opportunity cost is thus greater than the expected return.
Ah, but (price) inflation doesn't cause malinvestment. A money supply which is manipulated to cause inflation results in malinvestment. Assuming a fixed money supply, inflation signals that existing investments are averaging a negative real return. The correct response in this case is to cut back on consumption--a natural side effect of the rising prices--and look for new investments. If inflation is costing you 2% per year in real value, an investment which only loses 1% annually is a good investment (assuming you can't find anything better) and will actually bring up the average return.
If the inflation is due to manipulation, however, rather than negative economic growth, then that 1% loss may look good to an individual investor faced with a 2% loss to inflation, but it will bring down the average for the economy as a whole and reduce economy growth.
On this point I think we are in perfect agreement, though our interpretations of "productive investments" and "stable money" differ. I define "stable money" as money with a stable supply, i.e. free from manipulation in favor of inflation or deflation.