Any side chain can of course be implemented entirely without Bitcoin at all. Perhaps, some would not be viable as federated systems, such as those needing a true zero trust environment and wanting to also hold BTC in their chain.
Its not really a zero trust application if you are trusting an oracle. Those would be fairly esoteric though, probably most of the ones that are being seriously contemplated would be accepted with a federated system, with the exception of those that may be requiring extraordinary security. If it is something where folks don't trust that the federating entity isn't their enemy in disguise. It would be one trust removed to have the protocol change.
Without the Bitcoin network running the verification, why use Bitcoin at all though? Might as well use an Altcoin as the "separated currency".
It would be a better bootstrap to prove the case anyway.
Though some of the issues are insidious and won't manifest their effects until there are a lot of locked coin.
Federated systems work fine until the federated server (or the Bitcoin network) goes away.
Bitcoin works fine, until the Bitcoin network goes away.
I agree with all of the above and am happy to see that your thoughts are aligned with mine.
Why use Bitcoin? Simply because the altcoin as less interoperability as a "separated currency" and of course because it creates a new scarcity that does not respect Bitcoin's ledger.