Full disclosure, the last time I studied chemistry was 20 years ago, and I was not a particularly good student, so take this with a heaping helping of NaCl.

It isn't the direct reaction of Drano + PVC that causes the issue. Rather, it's the heat given off from the reaction of the clog and the lye. Apparently it's significant enough to be an issue. I tried looking up how much heat might be released by the reaction, but I went crosseyed reading the formula, so someone else will have to do the math on that one.

Also, I know you said caustic material doesn't react with metal, but Google doubts you on that front, for whatever that's worth. In fact, zinc is specifically called out as a metal with which sodium hydroxide reacts pretty strongly, which is important because many water lines are steel galvanized with, you guessed it, zinc.