Well, the GK's are pretty good at being where Daemons are about to show up and nipping incursions in the bud. But you can't nix Daemons by simply having the GK putting down an effective cordon all of the time or GK battles would be a case of "Successfully perform the ritual without any interruption".2. All greyknights seem to be able to do in the current book is banish the things, preventing them from manifesting on the world they are on, but not stopping them from popping up elsewhere.
That's not Daemons being eternal, that's Orks being trapped in a time loop of killing fun.3. When the demons crop up into fluff, their eternal nature is often played upon. Eg. The ork warband in the warp who get into an eternal fight with khornate demons.
Daemons are a plot device in fluff, they do what's necessary to A:seem like a terrible foe to overcome and B:make the hero look suitably bad-ass.4. Greater daemons are good. Very good. They are often referred to as being able to wade through marines (who can in turn wade through a hundred men each without flinching) till some brave named character can go give their spine a good punching.
The reason that they manage to wade through such huge swathes of noble warriors is that the warriors don't possess the necessary knowledge to deal with them, the correct runes and encantations, properly prepared specialist eqiupment, the daemon's true name ect ect. It requires specialists, who ya gonna call?
1: You don't have any data on comparative levels of incursion prior to Draigo's arrival, he could be doing very well indeed, the Imperium could be a daemon infested ruin if not for him, the Eldar could have entirely fallen to Slaanesh, if not for him, the Old Ones could have been destroyed by chaos if not for him....The "big badass champion" of the demon hunters, is... very good at fighting demons. So good that he ended up stuck in the warp, battling against them forever. Hes not actually getting anywhere, because he can't, theyre endless, but he won't give up, and will fight on and on, doing what he can to keep them in the warp, and not in the real world. He battles across the warp to get to the rifts where they spill into the real world, pops out, helps push them back, then goes to find another. And still, technically he's ineffective, as there is one of him, and presumably more than one incursion.
2: If he IS in the warp, there's nothing to say he has to only be in one place at any one time, it's the warp that obeys none of the rules of the physical realm, including Time. He could have destroyed rifts that predate the existance of man and ones that will occur long after mankind's eventual demise at the same time, neither of which will ever be noticed because, thanks to him they don't happen.
Such a weapon would be possessed of all kinds of baleful energies BUT be utterly absent of any of the problems with holding a part of a daemon for any length of time and expecting to get away with one's sanity intact.