I agree. IMO the only reasons to continue would be plot reasons which isn't enough. This worked so well because it was character driven and Oz had a complete arc that there is no coming back from.
It can't be The Sopranos where it's like Tony always trying to reconcile his two lives and dangling the mirage of redemption. Oz now has everything, and yet he has nothing. He's fully transformed into a true supervillain, I'm good with him just being a more powerful figure in the movies from here out. After seeing how great this was, I like the idea of several one and done spinoffs to flesh out other characters. Let's not oversaturate though.