I mean if we're talking half of *all* living organisms that mean's for Earth at least, it's ability to convert CO2 into O2 just got cut in half meaning global warming is about to drastically accelerate. I don't even want to think about the effect on micro-organisms, particularly the kind that tend to reproduce like crazy if there's nothing keeping them in check.
Honestly though, I doubt Thanos is stupid enough to not have thought of this so I presume his reasoning is that (as is the case after all mass extinctions) there will be a chaotic period of symbiotic system crashes followed by evolutionary adaptation such as niche partitioning resulting in a new equilibrium.
Problem is, that'd take thousands of years and most surviving intelligent species will have to rely on *more* technology and industrialisation just to make it through the strife. Some might even turn predatory, raiding other worlds for slaves to exploit the suddenly abundant resources they need to sustain themselves.