The landscape for wages in the PL is changing. With increasingly higher revenue from huge commercial deals all clubs are going to see a huge rise in base salaries for squad, rotation and starting players.
Comparable players at comparable clubs wages do seem somewhat relevant if you ask me because that's who we're competing with. We have similar revenue and similar wage budgets as a percentage of that revenue.
However, you are right that he's not earnt the money we have given him the past two contracts yet (though largely not his fault). But the judgement of whether or not he's earning his wage shouldn't be based on the stupid reasoning found in your original post but rather his overall impact on results (i.e. Do we score have a better goal difference when he plays).
But here's the thing *wages aren't solely about past and expected future performance. They are also about value, income and costs.
Walcott's last two contracts have been high due to the context of when we began negotiations, when he finally signed and the cost of replacing him merely as a squad player, let alone getting a better player (i.e. It was more cost effective to pay him those wages to stay than sign a new, comparable player and pay him less due to transfer fees).
Walcott's current wages also include ownership of his image rights by the club rather than him. So the final, rumoured, wage will look a fair bit higher than it otherwise would be.
The situation now is fine. We already don't fill our 25 man squad and we're gaining at least three foreign players squad players as Arteta, Flamini and Rosicky see their contracts expire.
I honestly don't understand why people care so much about player wages when we are more than rich enough to suck a few "bad" contracts. We have huge revenues with less than two handfuls of clubs globally superior, our wage budget as a percentage of that has a lot of room to manoeuvre and we have huge cash reserves.
Would you rather we:
Sell Walcott and sign an upgrade?