Yeah you’re talking about purchasing power and it’s true that even if BTC is worth $1m in [x] years it doesn’t mean that it will have the purchasing power of $1m today. 

However the adoption and finite supply are also positive factors that also increase the value (in purchasing power terms) of Bitcoin.

So in [x] years when Bitcoin is worth $1m in USD terms, will it have the same purchasing power as $1m today? No. But will it have more purchasing power than what $65k gets you today? Yes.

Does that make sense ?