Btrfs and Ostree definitely are different approaches to the problem. Btrfs has all the benefits you mentioned (cow filesystems and snapshots are just awesome) but it doesn’t support the system layers like Ostree does. So you can’t easily export a btrfs system into a container for example or rebase your OS on a new image.
To me both a super useful together too but covering different domains, everything mutable needs a backup and recovery method too, even on an Ostree system.
Stuff like guix and nix also cover a lot of these domains too (or can at least), but still leave stuff like user data as an exercise for the system admin to handle (though an exercise you can use guix/nix to configure something like btrfs to handle!).