its eight decimal places now. that can easily be extended to 16 (or whatever) or more decimal places as needed. so, no need for more than 21 million. just add more decimal places.
8 decial places is the smallest amount you can use in a transaction. So going to 16 dp would require code changes. You are just making more satoshis as opposed to more bitcoins.
thats the point. divide btc into smaller pieces as needed. a code change to add decimal places does not violate the 21 million hard cap, that stays immutable. adding more than 21 million btc violates the whole point of btc.. 21 million btc, and thats it. period.
as for a satoshi being the smallest unit right now, thats just a name. call the new smallest unit a microsatoshi (or whatever) or something. theres no real limit as to how small a unit of btc can be made.