The theory is that a mature, global Bitcoin network will have enough fee volume to pay for its own security. But is that a guarantee? What happens if Layer 2 solutions thrive and most transactions happen off-chain, starving the main chain of fees?
If the main chain is starving of fee and the fee become so cheap, why use layer 2 instead of layer 1? Layer 2 is fast and cheap but security wise its not that great and finality always depends on Layer 1.
The argument itself doesn't make sense, layer 1 will always take precedence compared to layer 2.