Since bcachefs no longer mainlined (DKMS) and therefore sits in the same class as ZFS, why would/should I migrate from ZFS to bcachefs?

The tiered drive setup is more featured if you're interested in combining SSD+HDDs. Unlike other cache drive setups like ZFS's L2ARC/SLOG and even bcache, the SSD drives in this setup are still usable space. Otherwise I wouldn't want to use a 1TB SSD as a cache in front of a 4TB HDD for example.

Another use case would be on a handheld like Steam Deck: internal SSD tiered with a microSD for unused games I keep installed 'just in case'.

I would even want it on my home NAS. Instead of a separate root SSD and all my files on an HDD ZFS array (maybe with smaller cache SSDs), combine it all into a single bcachefs filesystems. Maybe with a subvolume root that stays pinned to the SSD(s). And also being able to use the full capabilities of different size drive unlike ZFS because it's a home NAS for cheap. No erasure coding yet though, so I'm in no rush to migrate my home NAS

One reason is to use the Linux page cache instead of dedicating RAM to ZFS, given how expensive memory is now. I am very happy with MGLRU and won't miss ZFS's ARC.