I just paid you 21 sats from my minibits wallet. Fees were zero sats.

Over time, the value of those 21 sats will increase. The value of sats can increase, and dust limit for base chain UTXOs can rise based on higher fees. Not mutually exclusive. Both can be true.

That super high dust limit you describe applies to the base layer. Where it is absolutely something to keep in mind when DCA'ing. No argument there.

Dust limit for lightning is quite a bit lower. Occasional 2 sats on a 210 sat zap.

Dust limit for ecash moving in a circular economy is ...zero? Until you pay a lightnkng fee if you want to swap mints to a different circular economy.