Why would anyone even bring up the issue of the DM picking the monsters? How does this help an analysis of GWM/SS power?
The DM choosing monsters is how the game is played, so that information is brought up because analysis of the GWM/SS power requires all the variables - one of which happens to be the DM's selection of monsters.
On the other hand, bringing up the DMG CR rating recommended AC IS relevant.
Theoretical AC that may or may not happen at a particular CR because that's just not how that chart works (because something with 20 HP and a 19 AC could be the same CR as something with 80 HP and a 13 AC, and both can be CR 1, and thus face-able at the very beginning of a campaign) is relevant, but "the average AC is set by the DM's monster choice" isn't?
Make up your mind - either it is relevant to consider what AC you are going to face, or it isn't.
Beyond that, I present a way to determine
practical effect of the feat, and you are claiming that to be irrelevant because we can discuss
theoretical effect instead.
Let's review the waste of time/irrelevant arguments:
"The feats are optional - so just don't use them!"
That's a mischaracterization of a different argument, which is: Feats are an optional rule - so it's up to whoever chooses to use that option to make sure it works out how they want it to.
"Every table varies, so how can you say anything is imbalanced?"
That's an argument no one ever made.
"In my game we only fight skeletons"
That's not even an argument.
"It says in the rules that the DM can make his own rules, so just change the feat!"
That's not an argument either... and isn't the goal of someone complaining that the feat doesn't work as they want it to in their games to change the feat? It seems like you are saying it is a waste of time to remind someone that they can totally just fix the problem they are having - you know, stop all the threads debating whether the feat is or isn't overpowered, stop side-tracking conversations about suitable adjustments/replacements by using language that over-states "I have a problem with this" into "this is a problem for everyone" and invites disagreement, and just focus on talking about how to make a thing that you don't like into something that you do?
Mischaracterization again.
"You focus more on combat than we do - that's the only "problem""
Mischaracterization of the arguments that are a natural result of someone declaring a thing to be a universal problem - which is that someone not having the problem says "No, that's not actually a universal problem.", and usually because it just isn't helpful to anyone to over-state the scope of a problem, not for any kind of "you are playing the game wrong" dig at the person over-stating their problem as universal.