Argue all you want. When you've spent a few decades repairing the stuff, come back and revisit the topic. There's a difference between opening the cover of a diaphragm valve, making the repair, as compared to the replacement of an actuator assembly in a brass valve body. The article you linked repeats the essential point about PVC valve manifolds ~ "Female plastic threads on male metal threads are the most vulnerable to over-torqueing, so avoid them if possible." ~ edit that statement to "avoid them, period" and you have words to live by.
By the way, the plastic valves pictured above are based on the Richdel antisyphon valves, which have female-threaded connections that are slightly oversized, which will annoy the bejeezus out of anyone who fails to apply enough teflon tape (or plastic-approved paste dope) to the threads of the nipple or fitting that threads into them. For the price of said annoyance, you have a plastic valve body that doesn't a track record of cracking.
As far as seat wear (or sometimes just corrosion) goes, that can happen with brass valves. Plastic valves? Not an issue.
In the very long term, a possible issue with exposed PVC will be exposure to sunlight, which can result in brittleness.
One final possibility for having cracked PVC fittings, even if you did everything else correctly, is buying imported crap sprinkler/plumbing product from a big-box store, as opposed to American-made fittings from a supplier that has an interest in customer satisfaction.