For the migration to be complete, there would need to be a coordination between the btc network and an eth contract to move the issuing of the remainder of the supply over to the contract. And an agreement of how the new units would be allocated; it seems doubtful that this could ever take place.

I don't think there's any reason for a 100% migration. It would be impossible to get consensus for that, like you said.

Anybody who wants to move their coins over can. There are plenty of economic, technical, and moral (climate change) reasons to do so.

Besides, when it's voluntary, people can choose to lock their coins into whichever network they like Ethereum, Cardano, Polkadot. (note: ETH is the only network that has currently has contracts able to lock BTC)

The reason for a 100% migration is that Bitcoin ought to be switched off in its entirety. Which you can't do while it's being used.

I've heard some skepticism regarding the PoS of eth, so I'm in "let's see it when it's there" mode, but I'm somewhat optimistic here. Can't go fast enough for my taste, I'm really curious if it'll turn out to be what it's supposed to! Hopefully this will also mean people start to move over to this better tech.

It's been running perfectly in production since Dec. 1, with over $5 billion in ETH staked so far.

Migrating the legacy stake and EVM to it still needs to be done, but that's irrelevant to the consensus algorithm. It's just changing the data it reaches consensus about.

Interesting, I should look into this again and update my knowledge. Thanks for the reply!