If you ask Thanos, he'll tell you that his Infinity War plan is all for the greater good: wipe out half of all life, and the universe gets a fresh start. Unfortunately, Thanos' master plan just gets dumber and dumber the more you think about it. Mainly because the kind of population control he's after wouldn't even work.
While Marvel has maintained that Thanos is dispassionate but logical, and a moral stain he's willing to accept. Murdering half of all life in the universe may be seen as "evil" to some, but once Thanos can take his form of genocide-- sorry, rebalancing the universe to a cosmic scale with the Infinity Stones, he will retire. "And watch the sun rise on a grateful universe."
But that's because Thanos is dumber than either he or Marvel seem willing to concede. Not only would that plan not work the way he thinks, but would more likely descend entire worlds into nightmarish hellscapes of nature unbalanced. Allow us to explain.
Thanos Doesn't Understand How Populations Work
Before we get to explaining why Thanos doesn't actually understand how life or ecosystems are actually "balanced," let's just look at his logic in wiping out half of intelligent life. Since it's only the humans, humanoid aliens, and walking trees actually shown dissolving as a result of his snap, it's obvious that killing half of the people (or alien-inclusive "peoples") is what audiences are supposed to respond to. And at first glance, it makes sense.
Thanos claims that his plan emerged on his home planet, Titan, where overpopulation soon began to strain natural resources. They refused to cut their population in half, soon exhausting the planet, and perished. And so he went from world to world, culling populations by half. Because to Thanos "population" means "how many people are alive right now." Which is where his plan falls apart almost completely.
What Thanos doesn't consider, apparently, is how population... happens. As more people are born, mature, and give birth, the same rate of reproduction inflates population - and far faster than you might think. It took all of human history until 1804 just to get to one billion living people on Earth. By 1923, it had reached 2 Billion. In 1960, we hit 3 Billion. In just over the half-century since, Earth now sits at 7.6 Billion.
Because that's how population works. Even if half the people on Earth died tomorrow, the reasons we live so long remain (medicine, technology, etc.). So thanks, Thanos: you murdered 3.3 Billion souls on Earth alone, guaranteeing us a total of... 50 years before we're right back to where we were. Even if you double, or triple that amount of time for other alien worlds, it's not just nitpicking.
The plan enacted by Thanos is, plain and simple, a flawed one on its most fundamental assumption. But honestly, that's not even the worst way Thanos' plan fundamentally unbalances the universe.
Thanos Doesn't Understand How Ecosystems Work
Add in Marvel now claiming that Thanos killed half the plants and animals too, and he's cutting the resources in half - turning Earth into the exact kind of doomed ecosystem he claims he's helping to avoid. But going a step further, that means half of all plant or animal populations are sliced in half, no matter how evolved, intelligent, or even visible they really are. But even if that flaw is ignored, the plan is half-baked at best. Our knowledge of just what havoc that would wreak is limited to Earth, but it's enough to know that Thanos doesn't know that ecosystems develop a balance all their own.
When you move beyond "this eats that," and realize that lifeforms have different roles, the idea of Thanos wiping out half of the already struggling bee population alone would likely lead to catastrophic famine, no matter how many billions of people remain. Take out half the algae in Earth's waters - which produce 75% of the planet's oxygen - and it's possible that all life fails, with such a major force no longer capable of holding up its place in the larger system.
That's not even taking into account the microscopic organisms, bacteria, or unclassified life playing roles we don't yet know. Fans need look no farther than Ant-Man & The Wasp to see that in Marvel's universe, life thrives at every level of existence. And Thanos doesn't even know enough to keep the visible forms from collapsing. So at best, his plan throws every lifeform and ecosystem's structure into absolute chaos while guaranteeing that dominant intelligent species will return to their numbers in years, decades, maybe a century or two at best.
The master plan hatched by Thanos? It's not enough to permanently save the universe... but it's a great way of killing it.