The big problem with side chains right now is it kinda somewhat centralized aka remember me stating the federation?
The RSK platform will be launched with a Federation of well-known and respected community members (blockchain companies with high security standards). Each member is identified by a public key for the block checkpoint signature scheme. The Federation is able to add or remove members using an on-chain voting system. The conditions to become a Federation member (known as Federation Member Requirements or FMR) establish basic security policies and legal requirements that all members must meet.
The Federation will provide several services to the network. Some of them will be available at launch and other will be added later. Each member can choose to provide any of the following services:
Two-way peg with Bitcoin
Checkpointing services
However when Core( i think they will Blockstream is supporting side chains)adds special opcodes or extensibility to validate SPV proofs as a hard-fork, or to manage a drivechain, the Federation role as STTPs in the 2-Way peg will no longer be necessary. Until that happens, we expect the Federation and the sidechain validation to coexist.
And the for the checkpoints
By default, clients stop using federated checkpoints when if RSK hashing power is over 66% of the maximum BTC hashing difficulty observed in the best chain and the fees paid in a block are higher or equal to the average reward of a Bitcoin block. However, any user can re-configure this local policy.
So pretty much for rsk to be decentralized it must have 70% of bitcoin hashpower(luckily 90% has agreed to merge mine it so that part is out of the way) and core adding the protocol for sidechains then the federation will be deactivated