How it's impossible to recast purely cinematic icons

That isn't true. You saw the movie, loved Ford in it and then retroactively determined that Ford was the only one who could play him.

There were plenty of actors who could play it. We wouldn't even have a good measure of what a good or bad performance was except by comparing Ford's performance.

Granted, it's hard for me to imagine anyone but Ford in the role, but that has more to do with my lack of imagination than it does with the role being impossible to cast.