What if Chuck and Sarah had boarded that train in Prague? Some viewers say that season 3a (the first 13 episodes) are a useless misery arc and that Schwartz and Fedak also implicitly repudiate these episodes because they make Chuck and Sarah board the train in Paris in 3.14 Honeymooners as if season 3a had never happened.
I think these viewers miss the point. It is true that Chuck and Sarah board that train in Paris and run away together just as Sarah wanted to do in Prague. But notice what happens—they can’t run away from themselves, from their heroic nature. The very moment they toast their decision to quit the spy life, they are immediately prompted by their sense of duty to individually pursue the Basque terrorist and are turned on when they find out the other has been doing the same.
The very same thing would have happened in Prague. They would have been caught and they couldn’t really have run away from themselves because they are duty-bound heroes at heart. But if they have been boarded that train in Prague, there would have been a crucial difference: at the time, Sarah saw a dichotomy between her innocent non-spy Chuck (the thesis) and ruthless spy Chuck (the antithesis), the Chuck we see from 3.06 Nacho Sampler to 3.08 Fake Name, who appears to actualize Sarah’s deepest fears. By the time we get to 3.12 American Hero, however, Sarah knows that Chuck is now the synthesis (innocent spy Chuck), the best of both worlds, the third option she had never envisioned in Prague. She knows Chuck has been tempted and subjected to the ultimate moral test (his moral principles versus his spy ambition plus the allure of being with Sarah), and his moral principles have won. She now knows that he is never going to lose his chuckness, so when she finds out that he really wants to be a spy, she’s 100% turned on without any worries. If they had run away together in Prague, however, she would have still been worried that he might become a spy like all others (the antithesis to her Chuck).
Oh, one other crucial difference: Chuck did not feel like a James Bond spy equal to Bryce or Cole in Prague.
After 3.12 American Hero, he does.
And he is.

The nerd overtaking James Bond.
That is the reason the journey of season 3a is necessary.
3a was necessary, but that doesn’t make it any less painful to watch. After Chuck vs colonel, Chuck and Sarah getting together seemed like the obvious next step. If they managed to talk things out like they did during ‘vs the honeymoon’; if Chuck made his reasons for wanting to become a spy clearer and if Sarah hadn’t given up so easily we could’ve seen it happen sooner.
After 2.21 Colonel, Sarah was still leaving to work on the Intersect project. Remember what she told Chuck in 2,01 First Date, “CIA officers don’t get to choose.”
It was only at the beach wedding in 2.22 Ring that Sarah decided to quit the spy life and be with Chuck. And yes, that would have been the obvious next step if things had stayed that way.
But Chuck decides to become a spy and that changes things. The problem at that point cannot be solved through communication, but only through action. Chuck is an emotional person, we all know that, and emotions are a liability in the spy life. No communication can fix that.
The problem can at that point only be resolved through action, like in any good story. Chuck must decide to become a spy for the greater good (2.22 and 3.01), must realize his feelings are a liability (end of 3.02) and selflessly sacrifice his love for duty (3.03) and try to become a spy like all others (3.03 to 3.08), realize he’s losing his real self in the spy life (3.08) and decide to stay true to himself (end of 3.08 to 3.10), learn to master his feelings (3.09-13), have this resolution tested through the ultimate test (3.11) and pass the test, become a spy, pursue love again (3.11-12) and reverse Prague and put Sarah first (3.12).
This journey requires action and growth on Chuck’s part.. No conversation can fix this. It was the same for Sarah in season 2. She showed feelings for Chuck in 2.01 to 2.03, but her feelings became a liability (she went off mission and couldn’t shoot the Fulcrum lady), so Chuck had to pull back from her for her sake (end of 2.03). She had turn her feelings into an asset (admitted by Beckman at the end of 2.18) as well.
Chuck is much more emotional than Sarah, so the writers need to dedicate more episodes (3.09 to 3.13) to show that he learns to master his feelings and turn them into an asset.
Only then can Chuck and Sarah come together as spies with feelings (3.14 on).
That’s the journey.