It's two years later (than Does Ubuntu plan to move to btrfs as default filesystem?). Ubuntu is at 16.10 and we are about to celebrate 2017. (And there is also a ZFS wrench in the Canonical gears now.)
How much longer will ext4 remain the default file system? What will succeed it and when?
Chances are, ext4 will remain the default file system for a while.
I say this mostly because of precedent. Ubuntu tends to change very little 'under the hood' unless a major change is made in Debian Unstable, which Ubuntu is based on. (Example: The move from Upstart to SystemD)
Chances are unless Debian adopts btrfs as it's default filesystem, which most likely isn't going to happen anytime soon, ext4 will remain the default file system for a long time.
