Probably a good thing. He didn’t have the imposing physical presence to pull off Vader (behind the mask, his Death Glare would have been wasted), and I think Guinness’ portrayal of Kenobi as the quiet, wise instructor came off better than Mifune’s (likely) stern and fierce taskmaster would have.
Although, I will say that if he had been cast as Kenobi, at least the lightsaber fight at the end of Star Wars would have been more impressive than the arthritic swiping we ended up with. Criticize the “prequels” all you want, but the fight choreography benefited greatly from 30 years of Japanese-inspired action movies, giving us the far more fluid saber wielding we saw.
Actually, the voice of Vader was dubbed. I believe the actor playing Darth Vader was Dave Prowse, while the voice was supplied by James Earl Jones. TM’s voice was incredibly distinctive and powerful, and would have done very well.
Mark