[throwawayQuwcy]

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)

[JulianMorrison]

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.