Trees should really be harvestable for an effectively unlimited amount of shafts,
Really? Are you talking about just pruning branches and using those for shafts? I don't think you'd get many strong, straight shafts that way.
Footkerchief, I think there has to be a point in the game where we can safely handwave the vaguely defined minutae of what we propose is reality.
Trees have branches. Some will be straight enough, and long enough, to be used as shafts. That's not hard for me to accept, or to make the leap to branches being processed into shafts.
Maybe only some types of trees can be harvested? And maybe only one or two are useable per tree? The idea that it takes the time of a skilled worker, and a tree that can't otherwise be used as a log (and may only be harvestable for branches once a year, anyway) to make arrow and bolt shafts-for me anyway-is more than enough trouble to have to go through (with maybe a few reasonable further limitations, here and there, if there seems to be a need for them), without requiring the game track the angle, flexibility, and variation from true, of every single twig in the world.
As an option, we could keep the current method of still making shafts from logs, but that's a pretty wasteful method of making them, when Nature provides shafts that are already mostly complete, only requiring a degree of finishing.