Given that the financial crisis and the 2007-2009 recession are still likely to occur, the general consensus is that Kerry would lose to whoever the Republicans nominate.
That could certainly still be McCain. It could also be Jeb Bush. Or even George W. Bush seeking a second, non-consecutive term. The case for Romney is that it may be that neither Bush runs, and McCain's ideological and partisan heresies could prove disqualifying in a scenario where the Republicans are favored to win the WH regardless and where they aren't as concerned about electability. OTL, Romney *was* the establishment frontrunner for much of that cycle. However, his dreadful poll numbers, his inability to put away Mike Huckabee, and the fact that McCain was the only Republican who looked remotely competitive vis-a-vis Obama or Hillary brought about a late shift to McCain on the part of both primary voters and the GOP establishment.
So the thinking, at least for some, is that Romney might be better able to put away McCain if his opponent is an unpopular President Kerry rather than Obama or Hillary. Of course, OTOH, Romney had his own ideological heresies, and since the GOP would likely go after Kerry's foreign policy and national security record, McCain would be well-placed to capitalize.
I can see the case for either. Since there's no way to prove it ... ¯\_(ツ)_/¯