portions of time machine already have been (local backup uses snapshots, and is ridiculously simpler as a result).
Proper Time Machine "v2" is a bit more complex (basically a rsync clone based on local and remote snapshots), but still way simpler than the current "v1" method, which builds its own change table from fsevents and has to cope with a lack of snapshots in HFS+.
They can't use APFS as a Time Machine "v1" store because the unique feature of HFS+ to support directory hard-links did not carry over to APFS. Migration of a v1 to v2 volume would be a fun exercise in translations (folder hierarchy collapsed to volume snapshots, and differing crypto approaches for encrypted backups).
I suspect they are trying to get APFS in an evolutionary "stable" place so they can publicly document it with the iOS 12/macOS Mojave/etc releases - the changes they are making to support HDDs and Fusion drives this year are likely a big internal evolution.
I also suspect Apple is just not in a rush to move time machine over - it already works, has a surprising amount of integration across the OS for backup/recovery, and people tend to value their backups working vs using the newest technology.
Eventually, my belief is they will roll out Time Machine v2 and leave v1 (on HFS+) only supported for restoration. Just like with v1 today, non-APFS volumes will get a sparse bundle disk image to hold the APFS data. But they will wait until they have a strategy to migrate the external volumes and disk images.
Proper Time Machine "v2" is a bit more complex (basically a rsync clone based on local and remote snapshots), but still way simpler than the current "v1" method, which builds its own change table from fsevents and has to cope with a lack of snapshots in HFS+.
They can't use APFS as a Time Machine "v1" store because the unique feature of HFS+ to support directory hard-links did not carry over to APFS. Migration of a v1 to v2 volume would be a fun exercise in translations (folder hierarchy collapsed to volume snapshots, and differing crypto approaches for encrypted backups).
I suspect they are trying to get APFS in an evolutionary "stable" place so they can publicly document it with the iOS 12/macOS Mojave/etc releases - the changes they are making to support HDDs and Fusion drives this year are likely a big internal evolution.
I also suspect Apple is just not in a rush to move time machine over - it already works, has a surprising amount of integration across the OS for backup/recovery, and people tend to value their backups working vs using the newest technology.
Eventually, my belief is they will roll out Time Machine v2 and leave v1 (on HFS+) only supported for restoration. Just like with v1 today, non-APFS volumes will get a sparse bundle disk image to hold the APFS data. But they will wait until they have a strategy to migrate the external volumes and disk images.