I think you're right but unless something crazy happens the price is a proxy for purchasing power anyway. In other words, if it costs 1 BTC to buy a car you can be certain you'd also be able to sell your 1 BTC for $35,000.

That said, in a hyperbitcoinized world we would eventually reach a point where nobody wants to trade BTC for dollars anymore and therefore you could argue it no longer has a price in dollars (it's either zero or infinite, whichever way you want to look at it). In that case, purchasing power is all that's left.