Even before this rumour started doing the rounds, The Walking Dead bowing out after twelve seasons has always been pitched as the show's endgame.
Executive producers have gone on record in years past and talked about how the show always had rough plans in place that would take them right up to this point. In 2014 producer David Alpert was interviewed, and discussed the creative framework in place:
"I happen to love working from source material, specifically because we have a pretty good idea of what season 10 is gonna be. We know where season 11 and 12 [are going]... we have benchmarks and milestones for those seasons if we're lucky enough to get there."
Though at the time everyone shrugged off the suggestion that any end point had been sketched in, the recent rumours indicate a twelve-season run was always more than a hypothetical.