As for your first point, you're countering a fact (Messi assisted the winning goal) with a hypothetical (they might've won on penalties/maybe his replacement would've did something). You can surely see how this isn't a good argument?Maybe they could have won on penaltis, just like they did later against Netherlands. It wouldn't be the first time they do that several times in a tournament (1990, 1993 come to mind). Or maybe whoever had replaced Messi between Higuaín/Agüero, Lavezzi and Palacio (or maybe Tevez or Icardi if Messi's injury would have been before the WC) would have been able to create a goal. When you are able to not concede when it matters (and that Sabella team excelled at it), your chances of progressing are always high.
Yes, I would call one assist in 4 knockout games (450 minutes total) an unremarkable performance. Which is why it's good that this years performance has been so much better.
And for the second one, I disagree, because I don't judge performances based solely on goal contributions.