While Pieter Wuille believes that sidechains do not offer a useful compromise and add complexity, Jeff Garzik suggests that having aggregation chains that arbitrarily fork and then rejoin the main chain are both feasible and useful. Jeff Garzik’s off-chain transaction notions include federated sidechains with an economic incentive to not commit fraud using fidelity bonds which were proposed as a scaling solution. He also mentions that some kind of treechain like sidechain / subchains could be both a scaling solution and decentralized. However, he points out that no solid design has been proposed yet that is ready for production. Jeff Garzik also highlights that other than preserving mining decentralization/resisting censorship, things like (U)TXO commitments and fraud proofs have been known for ages to scale up blockchains.