How exactly are we defining the term "ledger" here? It seems that whatever percentage of what we now call the Bitcoin ledger that happens to be currently on a sidechain (assuming perfect 2wp forever and a perfectly secure SC) is simply being updated by that sidechain. That is, it is temporarily being maintained/updated by the sidechain platform rather than the Bitcoin platform, but the ledger itself ("who owns what") should conceptually be thought of as completely intact (insofar as the sidechain and 2wp remain sound, which was the starting assumption).

If you own 210,000 BTC, meaning 1% of the ledger (now known as the Bitcoin ledger, later to simply be known as "the ledger"), under these assumptions you continue to own 1% of the ledger no matter what sidechain you happen to be have your ledger stake being updated by at the moment.

Now certainly the "perfect 2wp" and "perfectly secure SC" assumptions are not at all a given. But insofar as they are, the ledger does seem to remain perfectly intact under such sidechains. Therefore, it seems to me that the argument against sidechains should focus on why these assumptions are not likely to be met (mining issues), which is a situation that will mess with the ledger (though only if people get duped into putting too much money into SCs).