There is a pretty prominent forum author, AnonyMint (his last incarnation was, I think, "iamnotback"), that mantains the view that pegged sidechains won't work.
Here mentions his doubts, but I admit that never really analyzed deeply his argumentation. He basically says the same like Peter Todd, that it's difficult to incentivize mining in a way security is guaranteed.
They can't possibly be impossible. Software and projects already exist which use two way pegged sidechains.
Is there a decentralized one among them? I'm very interested in the sidechain technology and above all working examples, but so far, I only know working federated sidechains (like Blockstream's sidechains and RSK).