Let's say Pop came up with a new offensive and/or defensive scheme for game 3. In truth, there's a continuum of possibilities that could've played out, but broadly speaking we can classify the potential outcome into three general cases:
Case I: The new scheme doesn't work and the spurs get blown out by 20 points
Case II: The new scheme is no more or less effective than the scheme we actually used in game 3, and the spurs find themselves within 2 or 3 points with a couple of minutes go go
Case III: We blow out the Mavericks early and the new scheme ends up being, in hindsight, a huge success.
Some people have issues with Pop's defensive scheme of going under screens. The fact is that there's no telling that "case III" would've necessarily played out had we not gone under screens. You'd need a time machine to play the game over again while implementing the new, "don't go under screens" scheme in order to determine that. And unfortunately,
we can't do that.