I would love it if a series I like would actually end, out of the blue, because then I would get some satisfying resolution to the fiction of the world. Instead, the more popular and interesting a series is, the more likely it is that publishers will want to keep making those games and milking the series, ruining it for me because eventually at some point the series and its fiction will start to get stale, and the series will let me down.
My problem is that no video game series will ever pull a Jerry Seinfeld, who chose to end Seinfeld when they were still on top of the ratings and everybody loved them. They offered him a zillion dollars, and he still walked away, preferring to "go out on top". Until certain game designers and game writers get that much creative control, video games will never have a moment like that.
Instead, every popular video game series is more like The Simpsons, where you merchandise like no tomorrow when the series first gets popular, then the series continues to shamble onward for two decades, largely on brand loyalty at a certain point after the quality has dropped considerably.
Halo 3 finished the fight, but that certainly isn't stopping them from making more games. Mass Effect 3 could've been a grand conclusion to the Mass Effect saga, for all time, but you can bet your ass they will make more games that say "Mass Effect" on the cover. It's fundamentally the same issue as the Star Wars trilogy; why just leave it, when you can bring it back from the grave and make dump trucks full of money as you make progressively worse sequels?
Imagine if one day, Nintendo said "Fuck it, this next one's the last Zelda ever, and we promise we'll actually try some bold new things with it", and they went all MGS4 on it, pulled out all of the stops, and actually concluded the Zelda mythology with one final confrontation, and a solid confirmation that those are the final Link and Zelda, and it is made clear what they do with the rest of their lives and what becomes of Ganon, Hyrule, the Triforce, and everything else. Hell, Wind Waker sort of actually did that! Except then Nintendo kept making more fucking Zelda games after that. So I guess Wind Waker is more like The Dark Knight Returns, in that it was an interesting conclusion to the mythology of the series, but apparently it was just for funsies and doesn't really count.
And despite me mentioning it positively in the previous paragraph, even Metal Gear Solid 4 couldn't accomplish absolute finality, because fucking Konami is going to keep convincing Hideo Kojima to attach his name to Metal Gear-related games until the day Kojima retires.
Sometimes I hate this fucking industry.