I've rewatched both S1 and S2 after CoE, and... I'm not saying something like CoE was necessary, but the I think the show and Jack's arc especially was moving towards a point where it had to be serious about actions and their consequences
*nods* Thinking about it, then S1 Jack is very much doing what he does 'because what else is he going to do?' (to borrow Angel's words). In S2 he finds reasons for fighting, learns to believe in the mission again (as it were), truly connects to those he leads. And then CoE is the past catching up ('We needed someone who didn't *care*') - 'All the Pretty Little Horses' probably being the best illustration. Oh! Or The Price of Commanding.