This is a good point--and also makes it clear why it's somewhat difficult (but not impossible) to change the number of decimals that make up the smallest unit on chain--it's not simply changing BITCOINS/100000000 (8 zeros) to BITCOINS/1000000000 (9 zeros).

antic

bitcoin

The Current Version of Bitcoin Source Code

SlackerStacker

No less important than halvings.

/** No amount larger than this (in #satoshi) is valid.
*
Note that this constant is not the total money supply, which in #Bitcoin
currently happens to be less than 21,000,000 BTC for various reasons, but
rather a sanity check. As this sanity check is used by consensus-critical
validation code, the exact value of the MAX_MONEY constant is consensus
critical; in unusual circumstances like a(nother) overflow bug that allowed
for the creation of coins out of thin air modification could lead to a fork.
*/

https://imgprxy.stacker.news/ziIFzuRXIneXDVoE46Vqa_Ntv4nfVsDGIgeX5KcgYz0/rs:fit:1920:1080/aHR0cHM6Ly9pbWFnZS5ub3N0ci5idWlsZC85NmNlMmY5OWQzZDNjOWIxNTRmZjhjZjFhYTkzMGVjMWNkYmI1OTRkYzZiMTIzYTJlMmQ3NDg2NWY1YWJkMWFjLnBuZw

https://github.com/bitcoin/bitcoin/blob/master/src/consensus/amount.h