I think the real problem would be ecosystem collapse.
Ecosystems evolve as complex, interdependent systems with nonlinearities. What happens when you kill off 50% of pollinators in a single instant? 50% of plankton? 50% of grasses? The problem with nonlinear systems is that killing off half of A and half of B won’t have a linear effect if the relationship depends on having minimum levels of A. Assume it’s a random function such that we kill off half of all plants and on top of that half of all rhizobium bacteria which fix nitrogen for many plant species. Now we’re killing off potentially all plants that depend on having a stable population of rhizobium bacteria, which will have a cascading effect throughout the already devastated ecosystem. It’s all about tipping points and sigmoid curves and such.
The truth is that it was a completely stupid idea, and it was what finally broke my love of the marvel franchise. Either you have runaway ecosystem collapses, or the populations will simply return back to their original levels to hit their ecological carrying capacities again. Kill off half of termites, and you’ll probably be back to the same level of termites in a decade or less. Even with people (using the word inclusively across all technological species), you’d have a population surge that within less than a century or so would be brought back to carrying capacities. Populations self-regulate via interaction with their ecosystems. You’re either going to end up with 100% extinctions or system recovery to current levels within a very brief period via normal reproduction and evolutionary dynamics.
It was a massive effort undertaken by an immortal and massively intelligent person that is inherently flawed because the marvel writers apparently never took Biology 101-102. I’m not saying it was GoT season 8 levels of bad, but after watching those last couple of movies I not only never rewatched them, but I checked out of the mcu pretty much entirely after having rewatched the previous movies multiple times each.