I actually kind of think that would have been a great place to end the entire series on. The monologue from Maggie at the end and the certainty that there was full blown war coming between the survivors was a nice ending point. This is no longer about zombies but about how bad everyone alive is, something the comics did a much better job of establishing earlier on. I don't think anything they could do coming up would build on where we are now. (and yes this was a terrible season)
And therein lies the problem with this show. It had a completely different tone and style than the comics and that's what made it popular. This is not saying the Comics are bad in any way at all, but this has been a major reason behind the show's downfall was when Gimple took over, he wanted to make it more like the comics and it backfired in almost every major way. You are correct in the fact that the whole "mankind sucks" motif kicked off almost within the first few pages of the comics. The show, on the other hand, made a point to go in a different direction early on, and barely touched that fact at all until they did a little bit in Season 3, but even then, they made sure to give the Governor a human reason behind his madness (losing his family because he was weak). Then they dabbled a bit here and there with some of the secondary characters, and then Terminus and Grady Memorial. But in the end, for the better part of 5 seasons, most of the premise was still firmly planted in man vs zombies, and it was definitely more thrilling and exciting than watching Negan do the limbo under invisible poles as he talks as much like a sailor that TV allows.
Much of this is also exasperated by the fact that the writers just cannot put any emotional weight behind any of it, for whatever reason, be it lack of actual skill or AMC meddling to keep costs down. But like last night. The whole garbage pail kids' betrayal thing just fell totally flat and was just stupid. The entire theme of the group sucked already and then to have them go "oops, just kidding, we no like you anymore" was just dumb and was blatantly forced tension. Add to that, a very horribly choreographed fight scene, and it barely limped over the finish line as passable (at least it wasn't a stupidly placed cliffhanger). And honestly, Negan worked at first, because dude bashed in two characters' heads that we had grown to love over 3 and 6 seasons, so that brought in a sense of dread anytime he was on the screen. Yet, all of that equity was squandered over the course of the next 15 episodes as Negan didn't do jack shit outside of killing a couple characters we didn't care a lick for in the first place and talk shit to people like Eugene. In earlier seasons, they were able to get away with shoddy writing on the mankind aspects, because it almost always fell back to "OMG Walkers!" Now that walkers are barely even background noise at this point, they have nothing to fall back on when the writing goes completely flat.