Just because Ford will be 77, doesn't mean he'll be chasing bad guys with his walking cane. He hasn't show any signs of being incapable of doing action scenes. I expect that if the movie gets made we'll still see him swinging with the whip, jumping onto moving cars and punching bad guys out.
Like any movie, It all comes down the story and the characters. If the audience is buying into that then Indy's age and how he looks will be a minor issue in the movie. I think that's where KOTCS failed. It wasn't because of Harrison's age.