This would basically make Bitcoin Keynesian, since coin stored in wallets would now decay with a given probability. So you would have to invest it at least a little to beat the decay (shrinkage) rate.
You're mixing up Keynesianism with money supply increases due to changes to the reserve ratio, discount rate, and printed currency. These happen to central bank controlled fiat money regardless of whether the people controlling the money supply are Keynesian.
Then you conflated losing some percentage of your cash assets due to inflation, which can happen even if the money supply does not change, to losing all of your cash assets with some probability. The former encourages investment, while the latter encourages not holding cash at all.
Once the primary way of gaining bitcoins is hacking wallets, the longer a bitcoin is behind the same private key, the longer that given wallet is a target.
IMO, the most vulnerable wallets are going to be the ones actively in use and stored insecurely, for example, on Windows machines subject to the recent NSA bug.
This would basically make Bitcoin Keynesian, since coin stored in wallets would now decay with a given probability. So you would have to invest it at least a little to beat the decay (shrinkage) rate.