Sidechains as they are currently implemented in Liquid make use of Bitcoin script features and some fancy programming that required absolutely zero change to Bitcoin.
Saying that the implementation of Liquid should have been decided through consensus is the equivalent of suggesting the economic majority should approve of every Bitcoin companies out there. It's pure, unadulterated nonsense.
Moreover the issue you refer to in regards to Mike's stubborn stonewalling of BIP65 is completely irrelevant to sidechains.
Mike was banned from the IRC chat room for being a dick and distracting the development process with his political bullshit.
Sidechains are indeed a complex matter as it stands at that is why they are not open yet to the public and general user. Once they are this complexity will have been abstracted considerably and their security vetted by peer-review. The lost bitcoin concern is not reserved to sidechains but mostly every service that does not involve purely peer-to-peer use of Bitcoin (web wallets, exchanges, etc.)
From an exchange stand point it might not be an improvement on transparency yet it definitely does not make anything worse. As for your inability to understand the importance of privacy in these scenarios it just reflects on your poor imagination.
No one has suggested Liquid, or sidechains for that matter, were scaling solutions so your are just attacking a straw man here.
Wrong, the Liquid sidechain, as implemented, required no change to Bitcoin. As for BIP65 and nLockTime (which is not yet implemented) it helps with a host of use cases (sidechains & Lightning being one of them). This is certainly not a function that is being pulled into Bitcoin Core only to serve sidechains.
You are working from an assumption that is wrong: implementing BIP65 is a whole different debate than sidechains. Moreover sidechains are not implemented into Bitcoin, they are created out of existing Bitcoin features and script language. They are a voluntary service that is not being force onto Bitcoin users therefore it makes absolutely no sense to propose that we need consensus to allow them to exist. What you propose works precisely against the permissionless aspect of Bitcoin.
So creation of web wallets and exchange require consensus?
So multi-signature should not exist because it is a complex use of Bitcoin's scripting language that has become an intrinsic part of Bitcoin?
No one is proposing to implement anonymity into Bitcoin, at least not yet, so you are again attempting to fight a straw man.
Some might do, not all of them. Are you aware that BIP65 is not yet implemented ?