Even on stock cars with low miles, there seems to be enough deflection in the bushing at the back end of the a-arm that the top of the torsion bar gets all of its paint scraped off, like this:
Under harsh conditions, corrosion can cause pits to form, and as they deepen, they can form stress risers, and ultimately the bar can break.
Also, like all springs, t-bars will sag over time. This isn't much of an issue, at least in theory, since you can keep adjusting the ride height to keep up with developing sag.
Brand new genuine Porsche front t-bars are available from our host for $110 each.