It is debatable whether the zweihanders were actually used in chopping pikes or merely used to push them aside and make way through them. If this was even remotely possible is because the very long pikes had the lightest poles of the polearms - you simply cannot hold forward a 4-5 meter pole if it is heavy. The torque will tire you in minutes if not less. Spears, halberds and long axes had a much thicker pole made of heavier, much harder, treated wood. Even if braced and hit perfectly with an axe these will still be a real challenge to chop in one go. This is not like splitting firewood, it goes against the fibers.
Extended damage did slowly chip away and weaken the wooden poles and it is very likely that they were replaced after the battle way before being in any danger of snapping. On top of everything, many polearms had extended metal bracers to the head that protected the wood from being damaged at all, thus saving the trouble of replacing them after battle.
Additionally, what I believe was probably more likely during the Italian Wars "pike-hewing" brigades was that a soldier knocked a pike tip to the ground, trapping the hastily made weapon, and then used leverage and gravity to snap the tip off the pike. I doubt that the looping of the pike's point was done in a Hollywood-esque fashion that some people are envisioning.
You bring up another good point: weapons were damaged over time... much like our upkeep system now.
