...
In one of the previous examples we had:
MC
| \
SC1 SC3
| /
SC2
Where SC1 had rogue devs/miners who inflated it with new assets or subverted the proofs so no SC2 would convert back to SC1 and so convertibility was lost.
Subsequently SC3 is set up to restore convertibility because SC2 had such utility that it kept going and new owner folks wanted to (somewhat) regain the convertibility aspects (including new issuance from SPV).
So now you have SC2 with some BTC convertible back to MC and some not.
Some people who put their confidence in something (sc2) backed by a corrupt value backing (sc1) would lose. Sidechains are not some magic bullet which nullifies end-user stupidity and provider scammery. It just makes it possible for end-users to have an alternative to scammery if they want to put in the effort to do due dilligence. That alternative is found missing in almost every other financial instrument these days (except arguably physical PMs.)