got another three (admittedly terrible) movies for his origin story
To which the Darth Vader I know and love (like the one in Rogue One) was not in the film at all. ROTS has murderous Anakin and then had Anakin in the suit asking about fucking Padme. That’s not Vader to me. Kid Anakin and angsty teen Anakin don’t fit the bill either.
looms over TFA by motivating its antagonist
Doesn’t even count.
and already had a 25 issue comic and a novel in the new canon
Which most people are not going to see. The more casual fans should be able to see a Vader centric story.