If an 18-player team wins the whole thing, that won't change the statistics. It will have won despite having less of a chance.
Well, we don't know that for sure. We're pretty confident at this point that more players = better chance of surviving the first 13 weeks. The smaller rosters may in fact have an offsetting advantage in the playoffs (I don't think they do, and no one's yet shown that they do, but they might). Whether or not a small roster wins this year won't really answer that question. If someone did want to answer that question, one way they might try is to show that a roster densely packed with expensive players (so densely packed that basically the only way to afford them all is by using the smallest possible roster) is most likely to score a ton of points over a three-week span at the end of the season. It's not enough to just show that having Rodgers, Chris Johnson, and Andre Johnson gives you an advantage, for example, because there are 60 such rosters remaining, only 27 of which are 18-man rosters. Big rosters have studs, too; all those 18-man rosters who think they've got a step on the competition because they own the most expensive QB, RB, and WR will still find themselves competing against bigger rosters that have all those same core players. You would need to show that the best possible roster for the playoffs is one that you couldn't afford if you took more than 18 or 19 players. This might involve correlations between preseason projections and late-season fantasy points, etc.
I don't think anyone's really going to do that, but that's basically what the stud-related arguments come down to for me. You can't just say studs give you a better chance of winning, because I can build a bigger roster with studs. I need to see that a small roster has some kind of advantage
just by virtue of being small. The higher-variance argument Doug brought up earlier is along these lines, although I'm not personally convinced it's enough. I think they're likely to have a lower average score than larger rosters, which mitigates their variance advantage somewhat. And to whatever extent variance does provide a real potential advantage in the final three weeks, I don't think it's enough of an advantage to make a small roster the right decision in August.