I used to think that about ReiserFS, too. It was in the mainline kernel, development was snappy, and it solved some performance problems. I used it all over the place.
Things then subsequently... changed. :-/
And I did move away from it.
But I had once expected ReiserFS to be permanent, especially since this Hans Raiser dude who was driving the ship seemed to be sharp AF, and this seemed doubly-true when his filesystem got mainlined.
But it was not permanent. It did not last forever.
This idea of permanence, or rather the lack of it, was the whole of the point that I was responding to and also trying to impress upon.
At the end of the day: We do not know the future. Things can change.
I've outlived this one high-performance Linux filesystem in my life that I was using. This does not in any way mean that I will be the last to outlive other Linux filesystems.
Permanence is not guaranteed.