Sidechains — assuming they work — peg the exchange rate. Similar to Lightning, it locks up X Bitcoin on the main chain and allows redeeming that X bitcoin. Who gets to redeem what part of X depends on the rules of the sidechain. As long as those rules don’t have bugs, sidechain coins should retain the same value as mainchain coins (maybe slightly less because it takes time to redeem and you never really, really know there’s no bug, or maybe more because it’s more convenient, or because there’s overhead cost as you suggest).
That said, there’s probably plenty of use cases where altoins are fine, especially if you don’t hold them over long periods of time.