Spiderchain is a remora chain thet would increase miner centralization(if the node is expensive/hard to run) while siphoning off txn fees from mainchain. Unfortunately sidechain txn fees paid to L2 validators etc does not secure Bitcoin from the cost to attack. Im not saying spiderchain etc are attacks on BTC but if they had some incredible fantastic magical features and took away traffic from L1 it would be bad. Drivechain blind merge mining is a potential solution to this. Regular merge mining is good too but BMM allows for new mining pools to spin up and compete far easier than if they had to tool for an expensive (potentially permissioned) L2 validator thing