That's a good point.
You seem to understand Economics. What's your opinion of we lose 50% of all BTC to a SC failure?
A sidechain that supports 50% of the Bitcoin economy would probably be as extensively reviewed, secured and protected than Bitcoin's own blockchain so it seems to me these type of scenarios are far out there.
I would much rather see such important amount of BTC on a community supported sidechain than on a centralized off-chain scheme ala Mt. Gox.