Could 1 Satoshi (SAT) be worth $1 one day?
It's possible, which means it happened either because,
1. Bitcoin is widely adopted
2. US dollar suffer from big inflation
3. Global financial crisis happened
3. Combination of above possibility
If that happens and usd is still worth anything, on chain transactions are going to be very expensive (1 dolar per byte, minimum fee)
Lightning and off chain transactions are going to be really needed, and on chain transactions are going to probably be used just by banks and whales.