I generally agree with what you're saying.

My original point was that in a similar way (using your example) when the USD price of bitcoin got cut by 75% a few years ago, I could purchase 75% less milk with my sats.

So while 1 bitcoin = 1 bitcoin, it does not always equal the same amount of milk. And milk is priced (across most of the world) in a local fiat currency, not in sats.

Therefore we are forced to do the mental math of converting sats to fiat to milk. And that is why the sats to fiat exchange value matters a lot, particularly if most of one's networth is in bitcoin.