Now I want to ask you people here one more time:
Why isn't an additional Proof-of-stake layer being discussed?
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In my opinion, an additional Proof-of-stake layer would be economically less intrusive. The reward could still go 100% to the remaining SHA256 miners. But the transaction fees could be partially distributed to Proof-of-stake validators.
If you mean something like layer 2 or sidechain, it's already exist. Here are few examples,
https://thebitcoinmanual.com/blockchain/spiderchains/https://www.cryptopolitan.com/mintlayer-layer-2-side-chain-of-bitcoin/In addition, some protocol (such as Babylon) let you perform staking on altcoin by locking your Bitcoin.