One of the interesting motivations for wanting the extensibility that sidechains provides is to be able to make that zero trust a reality for more bitcoin transaction types.
That would be great, if it was possible. However any given sidechain will always require more trust than the main chain. I don't believe the white paper made the claim that sidechains will be zero trust, did it?
The worst case scenario for sidechains is that they are used as an excuse to keep Bitcoin's transaction rate capped forever.
Based on the composition of the Blockstream team, I feel pretty safe assuming this is a goal which they will actively pursue.
In that scenario we do, in fact, end up replicating the legacy banking system. There will be a real Bitcoin which only a privileged elite can access, and the rest of humanity will be relegated to transacting in money substitutes.