You can use the 8-byte addresses earlier if you want, just requires your hosts and stuff in between to support v5, same story as with v6 there.
The point is that you aren't asking everyone to do nearly as difficult of a migration to support v5. Github would've been on it already, all the v6 pieces are already there.
Except you are, because all the same work needs to be done.
> You can use the 8-byte addresses earlier if you want
You can't have both this and "The 8-byte phase only starts when v4 has been abandoned" simultaneously. Decide whether you want a flag day or not and then be consistent about it. (And when you're deciding, bear in mind that flags days on the Internet are impossible.)
The point is that you aren't asking everyone to do nearly as difficult of a migration to support v5. Github would've been on it already, all the v6 pieces are already there.