Kirkman sayd he wanted to keep writing TWD until possible. He wants to take the story never-ending, because he always wanted to know what happens after the end of a Zombies Film. I think the AMC wants to follow Kirkman too. It'll be a very long time until the end
This is late but a Youtuber (cant rmbr who) said Rick probably lasted that long in his coma without having any water or food (which many use to support the coma argument) because of that Doctor from The Oath (Dr. Gale?). If you observe carefully, the room Rick was in when Shane tried to wake him up and the room he awoke in looked quite different. So he probably did wake up and live through those seasons.