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.
I agree that sidechains could introduce a "value-add by providing services which people need and cannot achieve on the main chain" and that this could increase demand to hold BTC.
What I'm not convinced of is whether the value-add is "huge" or not, and whether providing support for SPV sidechains at the protocol level is worth the existential risk it creates.
Let's see what happens with federated sidechains first...