I think they even if MuSig2 ends up getting merged into Bitcoin, for compatibility reasons it’s not going to replace MuSig1 in newer releases, it’ll just be added alongside it. Most of the whitepaper was too advanced for me to understand, but the OP mentions that MuSig2 reduces transaction fees, which means transaction data structures have to change. You can’t push an update that makes existing multisig wallets send a different transaction structure without making a hard fork, so I think that what will most likely happen is that an option to choose the type of multisig signature will be added to wallets.

This would still be a soft fork though, and unless there’s a way to “wrap” a MuSig2 transaction inside a MuSig1, it’ll take a while post-deployment before people sending MuSig2 transactions to each other becomes common as common as MuSig1.