Yeah, that's the weird thing. Comics often go on in perpetuity, but a TV show needs to end at some point, at least if it's going to be any good. According to Wikipedia:

"Executive producer David Alpert said the original comics have given them enough ideas for Rick Grimes and company over the next seven years. "I happen to love working from source material, specifically because we have a pretty good idea of what Season 10 is gonna be," Alpert said. "We know where seasons 11 and 12 [will be]... we have benchmarks and milestones for those seasons if we're lucky enough to get there."

If they're going to end the show and work towards that ending, at some point it seems like they'll have to make fairly major departures from the comics. Eugene would be a good opportunity.