Stable mean not more than a few percentage a day volatility. Otherwise the goods will be priced in fiat, not bitcoin, even though you can pay with bitcoin, the vendor will change bitcoin into fiat instantly, they will not keep bitcoin. That is bad for the value of bitcon.
There are plenty of 'real' currencies with way less stability than Bitcoin. Overall though I agree. I don't see how it could ever be stable, in fact it was specifically designed not to be with deflation built in.
The merchant instant fiat thing is a bit of a tried trope. Bitpay employees have said hardly a coin is sold on the open market. They already have buyers waiting for them. Whether that could continue if activity scaled up is a different matter, but that would also mean there were more buyers. Commerce is a bit of a side show for now.
Several percentage points of deflation a year is not big.