"It just is not practical to take a petabyte-scale filesystem offline to run a filesystem check and repair tool; that work really needs to be done online in the future."
It's also not practical sometimes to backup/format/restore just to shrink some much smaller volumes. It takes time to copy 6TB of data even over a gigabit link. The last time I tried to use XFS I got caught out because I didn't know that you can't just shrink an XFS filesystem the way you can with ext4.
Has this changed? If not, is it ever likely to?
"So, he asked: why do we still need ext4?"
The problem for me is that choice of filesystem is not _usually_ significant _enough_ for whatever I'm doing to research the differences in very much depth. I used to be more inclined to experiment, but got caught out too many times and ended up losing time on projects because of it.
It's not often very important to be able to easily shrink a filesystem, so long as you know you're not going to be able to do it in advance.
I pretty much know where I am with ext so that is always my first choice until I next encounter some particular requirement for best performance in some way or another.
So that really is the point of ext4 as far as I'm concerned...fewer surprises for people with other priorities. And I think that reasoning holds up well for choosing a default filesystem in distros.