Sure, but heat seeking missiles don't just plummet out of the sky when launched in the absence of targets without a sufficient heat signature. They can still be aimed at a target and destroy that target as intended. They just don't correct course without that thermal input (I imagine. I don't know much about military weaponry).
You can certainly make a case for magic missile behaving in this way. But for me, it is problematic because it requires a couple of assumptions:
1) There is a real and objective distinction between what is living and what is not.
2) The magic utilized by spellcasters is able to intuitively or automatically sense this objective trait.
3) If the distinction between living and not living is not objective, then the magic employed by a spellcaster is capable of making these judgments without input from the caster. Meaning magic has a sort of limited artificial intelligence.
For me in my games, these assumptions bother me. I typically like to play in worlds that are more subjective or relative as opposed to concrete and objective. Good and Evil are not clearly defined or objective realities, but rather very much exist in the greys that we know and understand. The difference between life and death, or at least life and not-life is likewise blurry and ill-defined.
In such a world, it doesn't make sense to have magic innately able to sense and thus define what is alive and what is not.