Well, to be fair my posts were directly responding to why this game (and others like it) don't seem to hit high sales numbers. I think it's because for some people who don't always have the money to drop on full price games, games that lack the ability to give sustainable gameplay (particularly if you're not interested in re-runs) and have a short one-time-through play time aren't always worth a 50 - 60 dollar purchase.
Admittedly, harder modes and topping your score are one way to increase replay value. Adding co-op is another. Downloadable content, online leaderboards, etc... are all just features one can put on a game to give it more reason to be played again. When any game limits the options or reasons to make a game replayable, whether it's by (bad) design choice or for another reason, it saps some of the value out of a potential game.
Madworld looks to be a pretty amazing game. I think it'd be a ton of fun to play. But when the question arises of "why didn't this game sell more copies", it's pretty easy to see that unless someone has money to drop on every good new game that comes out, they might find there's not enough play value for a full price purchase. Once you have to discriminate between which game you'd want to spend your money on at release, the amount of additional features becomes a bigger factor towards purchase, and it seems realistic enough to say a game I can't play fully with my friend when he's over, with my friend over an online server, and which I've seen the entire story in 4 hours may not have as much value as another equal quality game that I can get 10 - 12 hours out of the solo story, play with others to share the experience with them, or have different modes to completely change the gameplay experience.
Maybe the excellent gameplay, narrative and the challenge and desire to do repeat run-throughs give Madworld a replay value far and above its base 4 - 6 hours. But it seems like it's kind of a gamble to drop money on a game hoping that it'll be the exception, and not the standard, when it comes to those things (especially if you already are aware what features you prefer in games and don't usually look to the types offered by Madworld as a way to get more value for your money).
And just because some people don't think "length matters" doesn't mean, again, that others don't value it differently. When you cut out a subsection of the gamer population, don't be surprised when a game doesn't sell well (I, for example, have played a large number of the actions games listed as having 'equal length' stories with worse gameplay, but own very few of them. They weren't worth 60 dollar new price purchases, and get bought or rented when they're more in line with how much fun I think they're worth).
As for all the stuff about this being a critique to prove my "prediction" right, that just seems silly as I don't care whether the game sells well or not, just whether I'd enjoy it and whether I think it's worth a purchase. I was simply offering an answer, using my own experience, as to why one might not see huge sales for this game.