LN enables economy of scale. Imagine a future where the majority of txs use lightning (L2), and on-chain (L1) txs are much more rare. One L1 tx may cost 1mBTC. Or if you use L2, a node might charge you and 999 other people 0.001mBTC to route your payments (simplifying here). The amount of money hasn't changed, but it's spread out among more people.