But the opportunity cost of not changing your fiat to sats, so that you can buy things with fiat instead of with bitcoin is even bigger since you are not actively pushing the network forward.

In other words: from a "wealth increase" perspective it doesn't matter if you spend fiat or sats. For the fiat you've chosen to spend, you could have bought sats, but you didn't. So why not change fiat to sats, and spend sats so that the adoption curve of btc moves forward even faster.