As cool as that extended duel is (and IT IS COOL!) I don’t feel like it would fit in very well within the original trilogy from a philosophical standpoint.
Leaving aside both their ages and states of health and whilst Darth Vader would indeed be very interested and (judging by Rogue One) still quite capable of an intense Lightsaber battle Obi Wan has been off on Tatooine meditating on the nature of the Force for nearly two decades. He’s moved past having Mustafar style swordfights (see the Rebels episode Twin Suns for an official canon example or just watch how he handles Evazian and Ponda Baba in the Cantina - efficient, concise defence is the technique he now uses) and although it might be emotionally and visually satisfying to see Vader and he face off again the point, to me, is that while Vader has been stuck in a “brute force” state of mind and actually mocks Obi Wan for being old and weak he has completely missed the fact that there is more to learn about the power of the Force.
Yoda tries to teach Luke the futility of physical conflict in Ep.V and you can also infer that he might’ve been communicating similar teachings to Obi Wan during his time on Tatooine. When Obi Wan is facing Darth Vader on the Death Star he’s at once creating a diversion for the rest of the group and also letting the Force guide him on his path. He doesn’t know he’s going to die but he knows his destiny lies on a different path to the rest of them. You could also say that he might want Vader to underestimate him. He’s not there to beat his old friend again in a lightsaber duel, he’s been there, done that and learned his lessons. Vader hasn’t, and is approaching their reunion in a far less mature manner. Vader likely sees Obi Wan’s claim of becoming more powerful than he can possibly imagine as desperate posturing. All he’s seen of the Rebellion is a quick snatch and grab attack and lots of running away so when he sees his old master as a frail-looking old man he can’t imagine he’s anything other than weaker than when they last met. Having Obi Wan miraculously disappear in front of him through a gentle, focused and articulate use of the Force is a far more effective victory than simply putting up a physically good fight.
As regards the actual “fighting” in that scene - compare how Obi Wan faces Anakin on Mustafar or how Luke faces Vader in Cloud City - very different fighting techniques and mindsets. Vader is more “powerful” in each engagement but despite this every single time he gets beaten by his opponent’s emotional state rather than their ability to physically match him. On Mustafar he’s arrogant and showy and ultimately loses to Obi Wan’s calm and simple one-swipe move and on Cloud City he loses the confrontation with Luke because he misjudges his character - essentially putting him through what he himself was put through and expecting Luke to make the same choices he did, Luke chooses potential death instead. Even in Rogue One he doesn’t come out on top because the Rebels sacrifice their own lives to hand off the plans to the next person in line.
Every single time Vader has a fight in Star Wars he demonstrates a high degree of competence, skill and general Force strength but he never really actually “wins” any of them. To have a glossy lightsaber battle with Obi Wan where he ultimately bests him (albeit with Obi confusingly disappearing from Vader’s point of view) would betray that and create a contradiction that unbalances the philosophies of the story as I see it.
Apologies for posting an essay in your thread Ady, I just thought it was worth chipping in with my two penneth as nobody else seems to have brought it up.