If Bitcoin is going to be the world’s currency, a stock split style adding of place values past hundred millionth will need to be added to the main chain.
Discussion
Because self custody is the revolution.
More likely, those smaller values will be settled off chain. Lightning already supports millisatoshis.
Currently the cheapest on chain transaction costs hundreds of sats. This will increase significantly on a world Bitcoin standard. So there isn't much need for another decimal on chain. Nobody is going to pay 150 sats to settle a 0.5sat transaction.