Robert Kirkman is apparently discounting the possibility that ratings fall and AMC cancels The Walking Dead. Or, the expense eventually makes the show not worth it, and AMC cancels it. Or, and I think we all agree this is the most likely option, the world ends in an apocalypse that makes The Walking Dead both redundant and hard to produce.
Here's the deal: the comic book the show is based on is about 50 issues ahead of where the show is. Assuming that the show and the comic book continue their pace, it could take years for the show to get to where the comics currently are. And the comics aren't stopping.
In the most recent issue of the comic, Kirkman explained (via ComicBook.com), "It took us six seasons to get to 100. It won't take us six years to get to 200 and that will take us to season… TWELVE. And we'll still be ahead of the show at that point."
Yes, if the show continues on its current adaptation pace (and the comic doesn't end) there's still years and years of story left for The Walking Dead. AMC isn't going to keep the show around indefinitely. What's more likely to happen is that the show will a) condense things from the comics and get through more b) just go off on its own direction completely or c) get cancelled with no solid ending.
I'm hoping for something between the first two, since the show's highs and lows have been very distinct. It would be better for everyone if they mapped out a plan, stuck to it and worked toward a satisfactory ending for everyone.