OK, suppose the whole world used a trustless digital cash. Now you cannot do monetary policy. Now booms are booms and busts are busts and we are back to the early 1900s. Great.
Actually what would likely happen is that people would be incentivized to opt-in a digital currency with a monetary policy knob, and this would again become the de facto currency that everyone uses.
The problem with 2008 was corruption not monetary policy.
No solution is perfect. Come up with a better one. More regulation, however, is not what we need. We need to decide what we want the financial sector to actually do, and that probably doesn't involve a bunch of children playing games.
"Monetary policy" in reality simply means printing more money than is ever destroyed. It's human nature and always will be.
This means bitcoin's average price will go up forever when priced in fiat tokens. Or anything else for that matter - even gold's above-ground stock doubles every 35 years. How's that for an incentive?
The largest amounts of value will always settle in the best store of value for large amounts of value. Show me one with better fundamentals than bitcoin.