I think, that value is always stored in MC. You can only put your bitcoins into "escrow" (lock them in main-chain) and use "value" on side-chain.
Shouldn't you then believe that the severing of 2-way peg would result in a scBTC value of zero in all cases? I think I've convincingly shown that this would not always be the case. Therefore, a % of the value must actually be stored on the sidechain's ledger.
Sidechains introduce a huge marginal value-add by providing services which people need and cannot achieve on the main chain. My seat-of-the-pants estimate is that this will easily account for any increase in value associated with scBTC on an ecosystem level and some of the value will probably back-wash into Bitcoin itself in a big way.