Debating overlayfs

Anyway, the writable filesystem on top is not usable by itself. The data on writable disk is meaningful only on top of read-only disk(fs).

Wouldn't it be better, if COW file-systems like btrfs can provide a feature to write new blocks only to writable disk, instead of going for generalized solutions. Btrfs would need a way to check for the root of the tree(superblock) in the new disk, before using the one from read-only disk.