My plumber did some repair work on my main stack in the basement of my house. The main stack is made of cast iron, but he replaced a 3 foot section with PVC. I'm concerned the PVC will crack eventually under all the weight of the cast iron above it. Is this a valid concern?

There is no problem with having piece of PVC in place of C.I. - provided, the C.I. is properly supported above the PVC section. That pipe support will hold the weight of C.I. pipe.

If your plumber failed to properly support C.I above PVC - then your concern is valid. If that is the case, I would call him back and asked him to install proper pipe support

Milo