Stable sources of growth have existed for decades but somehow most investors aren't just going all in on those. That's because investors care more about good risk-adjusted returns than lame guaranteed returns.
>Investments would have to have very high upside to be worth a stable but lower rate of deflation.
Investments with risk-adjusted nominal returns above 0% are worth it if the currency is deflationary: even if the investment only has a nominal 1%/year return and the currency deflates at 2%/year, that's a 3%/year real return. So it's still better than just holding the currency, even though the nominal return is lower than the deflation rate.
> Secondly, I'm not sure how you can be certain that investment returns, without regard for risks, could be higher than the deflation rate.
Returns on most investments aren't certain in an inflationary environment either, so I'm not sure what's your point.
This leads to your first 2 points, where people might still want to invest to get a potential higher return than the base deflation rate, investments would drop compared to an inflationary environment. In the inflationary environment investments are necessary to prevent the loss of value, whereas it isn't in a deflationary one.