Iowa and NH are short news cycle stories. One just needs to be in the top tier
That is true but with that said...
2012 Romney (NH) (basically tied with Santorum, Iowa)
2008 McCain (NH) Huckabee (Iowa, McCain was 4th)
2004 Bush (incumbent, unopposed in Iowa + NH)
2000 Bush (Iowa) McCain (NH) - Bush was 2nd
1996 Dole (Iowa) (Buchanan won NH, Dole was 2nd)
1992 Bush (incumbent but was opposed in Iowa + NH)
That's 5 of 6 in NH. With the statistical tie in 2012, that's also 5 of 6 in Iowa.
In New Hampshire, Nixon won 68, 72, Ford in 76, Reagan in 80, 84 and Bush in 1988. In other words, that's 11 of 12 in New Hampshire, with the one exception being a second place finish by Bush in 2000. Clearly New Hampshire matters quite a bit. Iowa is a bit more screwy and can probably be largely ignored (beyond eliminating a bunch of low and mid-tier folks) but still it matters some just in terms of the numbers.
Granted, this 'system' is predicated on establishment funding. So it will be interesting to see where the money goes in this anti-establishment cycle. Trump doesn't apply to that in any event but he might win both or either and will likely finish no lower than 2nd anyway. In other words, he's gonna keep spending his own money regardless.
So yeah, the winner of either is not always the GOP nominee, clearly, but if you can't finish 1st or 2nd in either one, you're basically done. McCain finished 4th in Iowa in 2008 but won NH. Buchanan won NH in 1996 but Dole was 2nd. So basically after Iowa + NH, we will have a maximum of 4 credible GOP candidates, the 1st, 2nd place finishers in both, unless someone is 3rd in either and very surprising in SC (but surely no later than Nevada).
I'd say this comfortably...we will be down to 5 legitimate GOP candidates max by Feb 11th. And maybe just 4 (2 establishment vs 2 anti-establishment). Though, with the split in the party, I'd say 5 might be a better number. Especially considering one candidate doesn't need the money.
Iowa and New Hampshire will give us the 2 establishment candidates, that much I think is easy to predict. I count Cruz as establishment because he's a Senator and aside from baiting rhetoric, is enough of a 'company guy'.