Because he's a con-artist and the one thing that held those two together was running cons. Who said it would work out? I never said he gets out. I just said she should say that to finish the show. It brings those 2 full circle.
If I were a Hollywood writer Gus would have killed Walt and would be running the largest criminal organization in the world.
Gus didn’t have the guts to kill Walt. He had plenty of opportunities but didn’t, so he got got instead.
I can tell you how I'd start the spin-off right now. I'd start with the last scene of BCS, and do a 30 second rewind of the entire BB universe from then until some point where Gus had Walt on the ropes and let him off. Let one of the producers say as the screen is frozen, "What happens if, at this moment, Gus killed Walt? This is that story." Let it go from there.