Bitcoin becomes widely adopted, its marketcap surpass the dollar's, and it become so valuable that even a single satoshi is worth more than, for example, a couple of pens, a bottle of water, etc.
Here's a post from Ray Dillinger on this topic which you might find interesting:
https://bitcointalk.org/index.php?topic=819656.msg9170781#msg9170781Even in such a case where bitcoin becomes a globally adopted currency, a satoshi is probably still going to be just fine as the smallest on-chain unit. The bigger issue would be dealing with fees and the dust limit for transactions which are only moving a few sats. No point paying 100 sats to move 1.
That's where second layer solutions come in to play. No one is going to pay 100 sats to move 1, but would likely have no problems spending 100 sats to open a channel which could move 1 sat over and over again, thousands or even tens of thousands times.
Correct me if I'm wrong but couldn't this simply be done in the wallet side of things? As it's mostly just moving of decimal places anyway.
You can code your own wallet software to display any units you like. I could code my wallet so instead of displaying 8 decimal places it displays 20. Instead of sats being the base unit, now my base unit is picosats. None of that makes any difference to the protocol though, and the smallest amount I could work in would be 1,000,000,000,000 picosats, which is equivalent to 1 sat. Trying to change any of the 12 zeroes in that number would simply result in an invalid transaction which would be rejected by the network.