Yeah, I had similar thoughts. I was struck by the sharp difference between realistic behavior and the narrative necessity of the story. Realistically, Negan would kill the warriors and keep the weaker members as slaves. There is no logic in keeping a 200 pound cop who just declared his intent to kill you alive, and kill the 160 pound regular guy with a baby on the way.
However, the Walking Dead is Rick's story, always has been. All the other characters exist to give him something to love, mourn, and avenge. I noticed how they kept cutting to Rick's face as Glenn was being killed. It wasn't Glenn's story, it was the story of Rick being broken.
Negan's not looking for slaves though. He's looking for an already extant business to subsume. The weak slaves are easily replaced.