You're conflating chain splits with backwards compatibility.
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its not backwards compatible for those who do not move to the new signature scheme
Backwards compatibility with regard to forks is taken from the perspective of a node operator rather than a transactor, but yes. Perhaps a new term is necessary.
The simple question from a layman is : what happens to Satoshi's coins ?
If something like my migration BIP activates then either Satoshi upgrades the locking scripts for their coins or they get frozen.
While my research suggests it should be possible to enable a recovery option for BIP32 wallets, there's no additional data available that JBOK wallets would be able to use to prove to the network that they aren't a quantum attacker.