The security of the peg-out is backed by the greed of the miners. They are incentivized to mine the blocks and keep the system going with PoW. So it’s a bunch of game-theory and everything that currently keeps Bitcoin secure.
If a sidechain is successful, the miners get a steady income of actual L1 Bitcoin from it. So an L2 could be just as decentralized as BitcoinCASH (which is much better than Liquid or Fedimints), have increasingly better technology and privacy, and be completely backed by L1.