Q:

How Can Thanos believe his plan is sustainable? 

So, I watched Infinity War, and felt like Thanos didn't think things through enough.  His plan was to wipe out half of the life in the universe, but would that life not regenerate relatively quickly over the scale of things?  Did he plan on snapping his fingers every X amount of years?  How could he possibly think this wouldn't turn the entire universe against him?  Perhaps it will, and that will be the sequel, but considering he is supposed to be really smart, it seems like a weird solution to the problem.  Is Thanos really that short sighted? 

A:

Is Thanos really that short-sighted? 

From what we've seen of him, yes. 
He's known as the Mad Titan for a reason.  Sure, he may be highly intelligent, and sure, he may genuinely believe what he is doing is right, but none of this changes the fact that his trauma at seeing the death of his homeworld has blinded him to any other method than the one he believes is correct.  
He feels that if he had not failed in implementing his plan of culling half of his homeworld's population then his home planet would still be alive, and now he believes it is his duty to save every other planet from the same fate.  
This is his obsession, and it has blinded him to rationality to some degree.  Intelligent as he may be, his trauma prevents him from considering the deeper implications of his plan.