Kenobi - S1E3 (Spoilers)

I don’t think you would get that emotional journey with third sister as the main villian for example

Sure you could.

Make the catalyst for his return to a more hopeful outlook on the future be how his relationship with Leia allows him to see a chance to return the galaxy to the hands of the good. What's his relationship with Vader going to teach him? The only possible lessons are, "I guess it wasn't my fault, you made your choices," and "I can at least hope to kill you someday." Neither of those gives Kenobi a complete journey.

Kenobi's relationship to Leia and seeing in her a brighter future for the galaxy gives him something positive to fight for, not just a reason to give up a negative view of himself, but a positive mission to embrace.

Vader can't give him the emotional journey you're talking about. At best his encounter with Vader allows him to give up the crushing guilt, but it doesn't do anything at all to make him a positive force again. For that you need something completely different.

Again, Vader is not the antagonist here, and he's not capable of bringing Kenobi back to himself (or back to the Force).

That's Kenobi vs. Kenobi, and the struggle here is to find positive motivations for caring again. If Vader can't give Kenobi those (and he can't) then you're no worse off with 3rdSister.

Heck, arguably you're better off with 3rd Sister since she could renounce the Empire and become a hero, sacrificing her own life to help Kenobi get away. Because we already know Vader's future, he can't do that. But 3Sis can, if you want to roll that way.

3Sis is the superior foil for Kenobi in every possible way.