My friend also always runs multiple string lines through during construction. Later if a wire breaks or you want an additional cable or upgrade, you attach your new wire to a string and pull the string from the other end all the way through. If they had done this, they could have simply dragged a new fiber optic cable through.

You can pull the wire and second string together so you have always a single string in conduit.

Pvc also dries out and cracks. But yes.

If it’s underground (meaning not in the sun) the lifespan of PVC is 100+ years. It seems unlikely it will be his problem if it ever cracks.

PVC used in water pipes had some surprises, the lifespan there turned out to be less than optimistically expected when PVC pipes came to the market. 100 years might be hard to test for...

You can always splurge on some military grade pvc conduits :^)