That could definitely be the way the writers go with the show, ending after Season 8 or 9, depending. However, I see a much longer path ahead for TWD, one that transcends individual character popularity, ala Game of Thrones.
At some point Rick and the gang are going to find stability, either in overcoming whatever waits for them in Alexandria or surviving Negan and finding another solid refuge (I do not read the comics, so I don't know what has happened with the story line there). It's also clear that the writers are speeding up the events in the comics with the goings on of the show - we are already much farther along in the show than in the comics (if I've got that correct, this would be word-of-mouth for me). And, finally, the writers have no trouble switching out plot events to suit their needs, ala Judith/Bob/Terminus.
The writers could wrap up Alexandria, Negan and the Whispers by the end of Season 7 (or sooner), and iirc that would take them past where the comics are now. So what then you ask? Well, you've got a talented bunch of writers with a fantastically developed world in an apocalyptic setting all ready to go. Going out on a limb, I would say break up newly discovered characters from Seasons 6-7/8 and send them out on "missions" to find other pockets of survivors in an attempt to rebuild civilization. That would give the show a huge array of plots, characters and time to build multiple seasons upon, all outside of the comics.
Just food for thought.