Also, how do you decide which price inflation indicator to use? What we call price inflation is just the price index issued by the government. But Bitcoin has no government and the price index can be calculated in infinite ways.
Most viable would be imho a theoretical exchange rate against a basket of all major fiat currencies. But if you don't believe in them, that's probably not an option. Historians have a similar problem and afaik they often use a very basic and overall consistent measurement, the price of a cow.
I agree that Euros and dollars will have a more stable long run valu than difficulty, but this requires a trusted mechanism for reporting exchange rates to the system. I don't see how this could happen without centralization. If someone has a clever mechanism for incentivizing nodes to make truthful reports to the blockchain, I would be very excited by that. (nerdgasm) It needs to be relatively low cost and incentive compatible (truth must always be more profitable than falsehood). Good luck with this designing these systems is challenging/maybe impossible.