Perhaps Vader will be too strong, and Obi-Wan will have to fake his death? That could explain why Vader and Tarkin were so sure of Obi-Wan’s death. ROTS implies Vader knows Obi-Wan is alive… he just walked away, so Vader knows he’s out there. But a faked death at the end of the series could permanently get Vader off his trail and leave Obi-Wan to focus on protecting Luke.
And although it’s not 20 years in the making, 10 years in the making is still a lot for an epic duel.