Ext4 has been stable for over a decade. It's been a default filesystem on many distributions. It was the default on RHEL 6 which was first released over 8 years ago, and the default for the ext variants after that. It's been in use in Debian since 6.0/Squeeze or later, which was 2011. It's been in use in Ubuntu since 9.10, released in late 2009.
To be clear, your argument is that it's not a rare edge case to have a filesystem that was originally only in common use as the default variant 6-8 years ago or more for the vast majority of installations, which has persisted and since been upgraded?
In-place upgrades do have the potential to leave some non-default options for the final ext4 file system, such as 128B inodes instead of the 256B ones, which is where certain features like reduced timestamp granularity comes in.
I don't see what the user has to do with how time is internally kept on the system.