It's not just a liveness requirement. That is an overly reductive view. You incur fees if the channel party goes offline. The channel party can collude with the intended recipient (or at the time of a sensitive transaction) to fuck you at just the right time if they want to. Then the exchange rate can fluctuate before you can make a payment.
If you call Lightning trustless but not atomic swaps on the grounds of your rationale, it's plain old intellectualy dishonest.
