In particular, when we say the USD is worth 10% of what it was 60 years ago, we're not comparing that to any other currency. We're just saying that things that were $1 USD back in the day tend to be $10 USD now. Then when we talk about BTC being deflationary, we talk about 1 BTC a year ago being traded for $200 USD now (or whatever it is). It seems a bit apples to oranges to me.
So here's a question: when BTC value in terms of USD spikes, does the price of goods in BTC spike proportionally? If so, maybe that's a reasonable metric of value for now. I haven't bought in to BTC yet, so I don't actually know the answer. But in all these discussions about inflation or deflation and instability in a hypothetical BTC economy, does it really make sense to still talk about the USD/BTC rate?