As much as I like Negan, I don't think they can co-exist with each other. By the way, Maggie never forgave him as much as I remember. She let him alive, sure, but she wanted him rot in his cell.
But still there are many things that you certainly don't know:
First, in the comic, they never exiled Negan, he left by himself.
Second, the TV show no matter what they changed or will change, they always will follow from the source of the material.
Third, Negan's storyline is the only storyline that is still adapting from the comics and I don't think they will change that. To be honest, there not much left of his story, because we have The Whisperer War and then Saviors' uprising, which was already adapted. The only thing that is left is their final moments that they see each other. I think they will adapt his confrontation with Maggie from Issue 174 (in TV it will be second) and then he will leave us for awhile (probably in the first half or the second half of Season 11).
Fourth, Negan will never joined fully to their group, because for me, he doesn't fit there. His redemption story is very well written, and to be honest, he had more character development than Tara and Enid together.
Fifth, his time in jail have proved himself and prove us that people can change. Sure, the story can continue the ongoing thing by killing in the end the main antagonist, but that will repetitive and boring. In the story, there will be always a turning point, in this case is Negan. Also is partially because of the fanbase. I mean, there a lot of people that are fans of Negan. His character is unique and is still developing. Even Robert Kirkman himself like Negan.
Sixth, redemption can be given for everyone. For example, Alpha, I can see her changing from a lunatic to a lovely gardener. She is a psychopath from the beginning and people like that are rotten to the core. The Governor, who is a paranoid sociopath with to much hatred inside. Negan on the other hand, is not like that. As much as a dick he can be, he still have that humanity inside him unlike those two.
Seventh, the thing that Negan's life sentence was broken is particularly because Negan saved Judith and that is also good to mention that Judith made him change. Also, if you are against a dangerous enemy, you need any help that you find. Negan will be a good fighter against the Whisperers and he had proven a lot, for example saving Aaron or even Lydia.
And finally, does anybody have concern about the thing with Glenn's death anymore? To be honest, it's annoying. The characters have already move on, then the viewers should do the same thing.