I disagree that it works against that. I'd argue that that very premise of them only seeing the very beginning of the horrors, that their anixeties of the unknown have become so large and overwhleming, it makes that decision all the more rational. Not to mention we have no guarantee that the world is just completely better after that moment; the military has just cleaned up that initial area. They very well could have only been safe for a little bit. But it's that toying with what happens when you've lost all hope, act on that loss of hope, just to have that glimmer of hope return after it's too late that makes the ending so powerful IMO.
There's definitely still the uncertainty of how the rest of the world has been affected & everything, but given the fact that the majority of the movie took place near ground zero of the whole fiasco - and the fact that the mist was dissipating in the direction the characters were traveling, with miles of clear skies opening up behind the moving tanks - it can be inferred that the problem was at least placated at the source, which dampens that element considerably. Also the score for the ending scene felt really tacky in a horror setting, at least to me... it's whatever though.
Anyways, time to fire off some of the originals: L'Inferno (1911) J'accuse! (1919) The Cabinet of Dr. Caligari (1920) Hxan (1922) Nosferatu (1922)
And, for good measure, Nosferatu the Vampyre (1979) ---