Well there are people who claim miners are the custodians.
But I think that’s a lazy argument as miners are profit seekers.
If the don’t mine sidechain txs they lose out on business and will be outcompeted by a miner who does mine sidechain txs.
Discussion
The idea of a chain makes me instinctively reject it. It's not just stubbornness, a blockchain which plebs can't run node is nonsense.
Of course, I don't know the right answer of Bitcoin scalability, but I do know the wrong answer.
Bitcoin is the currency on the sidechains. You call them later 2s if that sounds better.
But I personally I am biased as I would love to have end-2-end encrypted btc transactions via Monero type sidechain.