I dont think this is accurate.
Three positive events (outs), combined with 2 negative events (baserunners), aren't equal to 3 positive events. For example:
1 + (-1) + 1 +(-1) +1
Does not equal
1 + 1 +1
Both mathematically and intuitively, getting 3 outs with zero baserunners in a one run game does not have the same "win probability added" as getting 3 outs while allowing a walk and a hit.
Your own explanation proves that (the "up slopes.")
WPA is worthless, but let's at least understand it.