The Main Reason Recasting Spacey Was a Great Idea
That terrible 'old man' makeup. Like Dave Bowman at the end of 2001 did the nasty with an uruk-hai.
Oh god it looked so poor and fake in the early trailers. It appeared to be possibly limiting his range of expression too.
All moral qualms aside, Spacey and Plummer are both highly talented actors, but Plummer was a natural fit for the role and Spacey was not.