Hale Vaccine. That still leaves with 10% of humanity left and the ecological damage. Still a grand finale, even if it leaves you unsatisfied.
So, this is more directed to the article in question, but this seems like spurious reasoning for the following reasons:
-The claim of "there's not enough scientists left to give info on the origins of the Chimera" isn't a strong one. From a purely in-universe standpoint, sure, there's a chain of logic to that, but from a storytelling perspective, info can be given to the audience through any number of methods, especially in a setting where telepathy is well established (e.g. Daedalus psychically contacts Hale, IIRE). There's countless examples in speculative fiction where information is provided through various means, and it's not always delivered to scientists (if anything, that seems to be the exception in fiction, especially in action games). Now of course, there's a case to be made that giving too much info on the Chimera could ruin their mystique (and I can certainly name examples, however subjective, where too much info on something DOES spoil the mystique), but then that isn't the claim the article is making.
-The attrition argument...sure, okay, but that also seems spurious. From an in-universe perspective, let's say that the Chimera can be wound down by attrition. The same also holds true for humanity, which by Resistance 3, have had their population reduced by 90%. The Chimera have the benefit of advanced technology, and having converted a significant portion of that original 90%, I'd wager they'd have the advantage of numbers as well.
-More importantly, from a storytelling standpoint, I don't recall anything in Resistance 3 to indicate that this is the case. The first thing we see in the game is Capelli and his community hiding (and then being forced to fight) a Chimeran death squad, and through the entire game, everything reinforces just how dire the situation is for humanity, to the point where Capelli has to deal with human enemies as well as Chimera. If the Chimera are indeed being ground down by attrition in Resistance 3, there's nothing in the actual game that indicates this is the case. Heck, if they WERE being ground down by attrition, then Capelli and Malikov's mission to New York is arguably a waste of time, or at least, a plan that might prompt them to wait awhile.
-Also, the issue of Chimeran losses...well, sure, the Chimera probably did take huge losses over the Chimeran War, but they still, y'know, won. In the real WWII, the Red Army took horrific losses as well, they still won in the end for instance. But that aside, what does the Resistance universe actually convey on the subject? From what I recall, Europe's overrun in a matter of months, and the Chimera are only defeated in the UK after most of the country's population has been killed or converted. After that, the Chimera are able to adapt to warmer temperatures, allowing them to sweep across Africa and Asia, and in Resistance 2, the United States loses, and loses terribly. No matter what losses the Chimera might be taking, everything that I've ever seen/read in the Resistance IP conveys the idea that the Chimera are basically unstoppable, and that like many 'horde type' enemies, losses are irrelevant when they outnumber their enemies to the extent that they do (and have superior tech to boot).
They either don't care now (Sony definitely doesn't give a shit about KZ & R and might as well not exist for them) or have different priorities now. Resistance 4 would be pointless, and they would more or less have to reboot. They're more focused on Spiderman, Ratchet & Clank, and the upcoming Wolverine.
I doubt you'd have to reboot Resistance. You could set Resistance 4 in the aftermath of the Chimeran War, dealing with taking back the planet (or part of it), unlock further mysteries while doing so, etc. In essence, actually play what Resistance 3 covered in its credits. Heck, part of the reason the pitch for Resistance 4 was turned down was because of its post-apocalyptic nature, and the fear of overlap with stuff like 'The Last of Us' and 'Days Gone.'
I meant by gameplay terms. While some new stuff was added, it was the same: two/three weapon limited load out, and get behind cover to regenerate health. I know GG changed the color palette, which most had no complaints about and the level design was a bit more open, but it was same old, same old. I'll give credit to Shadowfall for a least attempting something different with the story and characters, including featuring more grey areas, but it wasn't enough to keep the series going. I know the game is being vindicated by history with fans, but why they chose to do this now, and not have the opinion back then is up in the air and varies from person to person.
The original Killzone already had a 3 weapon limit, but yes, KZ2 did introduce regenerating health (I think?) and a cover system.