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.
proof of work wont make it to the mainstream.
1billion
TM people will never benefit from it ~directly.
its insane. they just need ~apps.
ETFs, sidechains, offchains, blythechains.. whatever.