The tree exists only because the wooden spear is its own craft. Most of the time you skip that step: the spear you have been poking boars with goes into the recipe as it is, worn or not, and comes out as a fresh stone spear. That makes the upgrade a cheap repair too.
Stones are never the issue; the first node you hit covers them. The real cost is the wood, which you also want for your first walls, so don't craft spare wooden spears just to upgrade them later. The craft takes seconds and no bench, so do it in the field.
Once a metal blade turns up, a salvaged sword lasts far longer per craft. The spear keeps its place as the thing you throw at a running boar.
