On-chain transactions don't understand smaller units than satoshi. LN can do mSats too.
But imho it's all a convention. Right now people work with Bitcoins, mBTC and satoshis.

My math tells that no matter how many zeroes are after the decimal point, if they're all zero it's the same number.
But I will add that in order to indeed have this in bitcoin - ie on-chain - bitcoin core may need changes and that's imho not necessary now, as long as 1 sat worth less than 1 cent.