Clinton (But was unopposed and incumbent president)
Harkin (Did not win nomination)
1988 Gephardt (did not win nomination)
In only three of the five did the Iowa win go on to be our nominee and in reality, you can't count 96 because the outcome was never in doubt and the caucuses weren't contested. Even 2000 is an iffy example, given that fact that no sitting VP has ever been denied a party nominatino and Gore was the prohibitive favorit in terms of polling, money and endorsements. Really, 2004 was the only time that winning Iowa gave anyone valuable momentum to the Democratic nomination.
One, general election polls right now mean NOTHING. At this point in 91, Bush's daddy looked invulnerable and our eventual winner hadn't even declared yet. At this point in 99, W looked poised to win a Reaganesque landslide against Gore. Instead he lost the popular vote and truth be told, probably lost the electoral vote as well. In 88, a lot closer to the general than this and with the nominees actually decided, Dukaksi led by double digits. Also 7 points isn't "losing badly" even if this poll is accurate. Even if this poll is accurate, the race could be dead even with the MOE (although to be fair it could be less close too) and its only one poll. Others show a tighter race.
Basically, the political climate is so hostile to the GOP that it's hard to imagine them winning much of anything, even if our nominee is Hillary. Indies aren't going to break in favor of a party that is losing ground.