Knowing Vader, why would he ever presume Obi-Wan’s death? Next to Yoda, Obi-Wan is public enemy number 1: the only surviving Jedi Council member, and who Vader has a deeply personal vendetta against. I doubt Vader and the Empire would let his escape go. I imagine there’d be wanted posters everywhere and the moment any Stormtrooper is able to locate him the leadership on Courscant would instantly be aware. This isn’t Cal Kestas or Kanan Jarrus.
For that reason, I find it more likely Obi-Wan and Vader fight again, and this time Obi-Wan will find some way to fake his death. It would explain a lot – why does Vader and Tarkin presume Obi-Wan’s death? He faked it somehow, permanently getting the Empire off his trail and allowing him to focus on protecting Luke. Why does Obi-Wan know Vader’s “more machine than man now” when he last saw him burning alive? Saw him personally, and realized that Anakin = Vader in the suit. Why didn’t Vader ever trouble to visit his old home on Tatooine, the first place to visit if you were hunting for Obi-Wan? His death was faked so he never bothered.
A fake death could even tie into the “death” of Obi-Wan and the “birth” of Ben.