I think it is not absolute value what defines a bubble.
It should be relative.
E.g. perceived value of an asset compared to it's real value/revenue/growth.
Prices only remain flat (0% inflation) if no increase in productivity occurs.
2% inflation doesn't mean the currency has been debased by 2%, it means: if it has become x% cheaper to produce a basket of goods -- and the price of this basket of goods has increased by 2% -- the currency has been devalued by (2+x)%.
Producers are constantly competing to cut the costs of production, in order to gain market share at the expense of competitors, so I don't see how flat prices can be a reasonable assumption.
Increase in production increases supply, so prices fall -> deflation.
Yes, inflation can have other reasons than debasement (for example, productivity decrease) but this affects both types of "money" - fiat and bitcoin, so I left it out to keep it simple
Cash real interest rate: 0% interest - 2% inflation through debasing currency = -2%
BTC real interest rate: 0% interest - 0% inflation = 0%
Note how this benefit of BTC is not bubbly by itself (growth expectation usually is - due to it being based on past growth)