The format gets a new major version precisely because it is incompatible.

While the name bzip3 itself would be okay for reasons others stated, I still don't like it because it is so easy to confuse with bzip2 and may indicate a false relationship with bzip2 regardless of intents. A spiritual successor doesn't have to be named confusingly similar, after all.

If they have to be incompatible then it's better to not conceal that. Generalized file formats require you to implement more stuff to support them, and we can't tell the format by looking at the file name.

this is the way.

The ext4/3 filesystems, notably, can read/write ext2 (and for ext4: ext3) filesystems in a compatible way.

IIRC, ext3 filesystems, if properly unmounted can be mounted with an ext2 driver

Oh yeah true

Perhaps not the best example