If we're looking at 'future' filesystems, is fragmentation really an issue in an SSD world? Not that there isn't a lot of spinning rust (and will be for quite a while), I don't think it's unreasonable to assume "most block storage is going to be SSD in the future" when allocating resources to priorities.

Fragmentation in ZFS wastes a lot of space. So it doesnt matter if it's SSD or not.

The day I can furnish my NAS in SSDs for roughly the same price as HDDs probably won't come before any current filesystem is obsoleted for some reason or another, methinks.