Well, he never wanted it to go longer than 4-5 seasons anyway....he's said that for years. Let me break it down.
"Mad Men" renegotiated their contract for some insane amount of money, leaving very little for "Breaking Bad" and "The Walking Dead". Which is one of the reasons Frank Darabont was fired from "The Walking Dead". And AMC owns that show, so the writers, etc. had basically no leverage to negotiate.
AMC tried to pull the same shyt with Vince Gilligan and "Breaking Bad", and he basically told them "fukk y'all, I'll take this show to another network"....because Sony owns it, not AMC. After that, AMC asked Vince how many eps he wanted, and they came up with 16. So he got what he wanted.
Fred.