I cannot see any noteworthy economic disadvantages with sidechains.
I can think of a big one. You cannot use Proof of Work on a sidechain. Therefore you accept security and/or counterparty risk. It will up to the sidechain user to determine how much risk to accept.I think Team Sidechain sees that as a feature, not a bug.
It's also one of my reasons for believing BTC's antifragility will endure and win out or equilibrate versus the 'free options -> hollowed out mainchain' scenario cypher describes.