It’s only a movie. Personally, I get about mid-way through any of these apocalyptic movies before my mind starts wondering and I think about all the implausible elements of the plot. In the case of the last “Dawn of the Dead”, it took about ten minutes for that to happen.
I can certainly understand your thinking, even if I don't agree with it.
Personally, I think the "I am Legend" story fits in with a newer type of literary artwork: one that starts out as a neat idea, but grows by accretion into a phenomenon.
The best example of what i'm talking about is probably Star Trek.
Clearly, the goofy little Matheson story from the fifties has grown into something iconic.