> if someone wants to maintain their PR score, they need to actively engage

Wouldn't that encourage people to "actively engage" even when they have nothing to say just to keep their score?

Goodhart's law comes into mind here. People who want to stay relevant on this site will take your measure as their target. There's no system that can't be gamed if you know the rules. But the more "rock solid" you try to make those rules, the more you embed intrinsic discrimination between participants. You put "Proof of Work" in the rules and you alienate newcomers while all the rest do "busywork".

Your own comments and submissions would sink like a stone because you didn't engage enough, regardless of how interesting your points are.