Will the lack of privacy on Bitcoin enable state co-option?
BIP 300 has nothing to do with privacy, sure you can make privacy focused sidechains, but we already have liquid and no one uses it.
Will the declining revenue for miners mean bitcoin transactions take weeks to settle, forcing users onto other chains?
This makes no sense, the difficulty adjustment exists
Will the scarcity of bockspace on Bitcoin force most users to rely on a custodian?
If this is the scare, something like coinpools is a much better approach. It actually works on solving the problem rather than just creating another blockchain.
Maybe no one uses Liquid because
  • it's trust model is worse
  • there is basically no tooling or software support because it is seem as a Blockstream thing, not a community thing
  • it doesn't actually provide privacy out-of-the-box, you need someone to host a coinjoin pool inside and there are none -- whereas the zSide sidechain would automatically be that perfect ongoing coinjoin with no overhead, cheap fees and great UX
  • because you have to do KYC on an exchange to withdraw
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something like coinpools is a much better approach
I agree with this, I would actually use a coinpool whereas I wouldn't use a drivechain. Coinpools are non-custodial and kind of like a lightning channel, just with more than 2 keyholders. But I also want drivechain to be an option for people.
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Coinpools are those things you need two complicated new opcodes to implement and even then they still basically do not work as you need every member of the thing to be online for anyone else to transact?
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Yes, it's very exciting!
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