In theory, to me, the number should not rely on how many plays are hit to the player or how many the RF'er takes away from the CF'er, if it is meant to determine how good the player is.
However, the number is meant to capture what the player has done, and if the RF'er makes the play he could have made, he shouldn't get credit for just being there. It's like saying, "I was on deck for the walk off and should get some sort of credit for not getting a chance."
Is JBJ really a worse defender because Betts gets to balls he could have caught, or because our staff Ks more batters or induces more GBs than other teams? Or, because there's this huge wall in short LF-CF that stops him from making some catches other CF'er make,because they play in a bigger park?
I can see the argument being mad, and I think it has merit, but WAR is meant to capture what you have done not could have done. It places value on HRs vs singles or 2Bs. It places value on many factors all at once, something that is hard to do at the water cooler armed with 20 different stats.