I didn't know that.... Lee is one of my favorite actors. And actually, I think he would have been very, very good in the role. However -
Shaw was perfection. His performance cannot be topped. It can't even be equaled.
We discussed, (or similar), before... .there are a few performances that simply could not have been improved on.
I would say George C Scott in Patton. Gregory Peck in To Kill A Mockingbird. Joan Plowright in nearly every role she ever played. I'm going to mention what could be one of the most underrated performance of all time - Sandra Bullock in The Blind Side... not a super great movie but her performance was amazing. Brock Peters in To Kill A Mockingbird (just possible my all time favorite).
And, of course, quite a few others.... but still I think a very limited category.