As you say, people can always factor in a consistent level of inflation or deflation in their planning. That's easy.
For deflation it is not as easy as you think. You point out unpredictable deflation leads to things like the Great Depression. The problem is that predictable deflation also leads to recessions as well. Inflation destroys wealth, which is the value of past investments. Deflation destroys the value of future investments, destroying the value of current investments.
Nominal Interest Rates = Real Interest Rates + Inflation/(Deflation). The higher the deflation the lower the real interest rates are. The value of investing in real asset fall so investments in future production fall. The value of investing in financial assets increase, shifting more money to those already wealth and less inclined to invest in tomorrow.
Take a look at what happened to Great Britton during the interwar years when Winston Churchill deflated the money supply to bring the pound back to the hard money standard – a 10 year deliberate, predictable, and well announced plan. Investment sank, productivity gains declined, and a general recessions was had. I would also point to America between 1870 to 1910 and Japan for the past 20 years. While less predictable, times of steady deflation, lower investment, and recsion.
BTW, in a relativity steady inflationary environment you don’t need to find investments that beat inflation. You need to do that for fixed financial assets, like cash or bonds. Real assets (factories, homes, college degrees, stocks, patents, etc.) value tends to increase at the same rate as inflation.