It is possible all L2s must necessarily be non-final to maintain the value of on-chain settlement. That idea has crossed my mind. If everyone is comfortable with the security of the L2, and there no cryptographic or security need to settle on L1, and it is effectively free to settle on L2, them there is no longer a need to mine in L1. Of course this breaks the entire system because L2 security depends on L1 security which is guarded by sufficiently large hashpower. So a "perfect" L2 can never actually eliminate the need to settle on L1, and L1 necessarily must be "more secure" than L2 alone.