That is the thing, we can, and have done so. The war was spread across 2 whole shards. That is a conflict of magnitudes wider than if you even include the whole planet in a war. We are just plainly too powerful a threat, even if we cannot be everywhere you buy only time before we show up and just end it.
As for Zenos, he was just given power up after power up with each death, or push back. Whether it was a Dragon eye, Reaper powers, or just the plain fact he was brought back from the grave he was just propped up by whatever. I was happy with his introduction, but when his depth was "Fight me" he became a boring antagonist with no depth and essentially became immortal as he died and was dragged up from the grave.
Ascians were dangerous as they had the immortality to give them the time to study, plan, and repeat the process non-stop. Beyond that, their literal capability to think something into literally existence proved to a slip up to why we even exist in the first place. Literally we are a byproduct of their failures to put into perspective how potent a threat they are.
So the issue is, how do you fill the gap and make a threat capable of being a problem to someone who can kill Primals/Ascians/Anything (throw an antagonist we've bested them quickly)
