If our expectation is that fiat is dying and will be replaced by bitcoin, then it makes sense to keep your bitcoin and trade as much fiat for truly valuable stuff as possible.

I think of spending bitcoin as something like activism. I want to get to bitcoinization faster and I'm willing to support bitcoin merchants because it helps us get there.

If there's a bitcoin-only seller, I'd definitely consider buying from them. If it's someone who's just going to immediately exchange the bitcoin I pay them for fiat, then I'd rather keep my sats and short fiat by taking a mortgage.