the whole point that Peter R is trying to make is that with SC's, value gets "shared" btwn MC and all SC's. with the potential of being "severed" or "fragmented".
we don't want that with Bitcoin. we want it ALL on the mainchain. we want all outsiders to be forced to "buy in" to BTC for their seat at the table.
Peter R has emphasized the potential you suggest is not quite realistic.
There is again a fundamental misunderstanding in your comment. At present, a important amount of value is sitting on exchanges for example and while you would like to argue that these BTCs are on the chain, the point is the value for the user is not. It is detached and in the hands of a counterparty which could easily corrupt it, inflate it, or simply run away with it.
Sidechains does not change the dynamic to enter the economy. Your notion that someone could simply create a sidechain and "siphon" BTC is possible but does NOT grants them a position in the economy per say. Unless they decide to steal their sidechain's users units then they have no control over them. Exchange operators and other controllers of off-chain schemes can access their users funds in the very same way. Sidechains do not introduce that problem