In order for the series to retain canon, in the final duel Vader must see himself as the student. For him to even admit that, Obi-Wan would have to completely destroy him.

Obi-Wan literally cuts off all of Vaders limbs (including arms)? Vader is already mostly machine so he will be immobilised but will survive. Then before Kenobi kills him Qui-Gon speaks to him or Palpatine intervenes to save Vader...

If Palpatine uses force lightning and shocks Obi-Wan this would explain why he looks older in Rebels S3 and ANH. It would also explain how Obi-Wan knew he was 'more machine now than man, twisted and evil'. 

I'm really happy with this theory as it fixes up many canon discrepancies... thoughts?