I find those Harrison Ford purists to be quite odd.
Indiana Jones was never some real terribly complicated character, the films are all about the adventure. Indy is there to fill-in for the viewer, it's like a video game and he's the player we control.
The appeal of these films is that, secretly, we all want to be Indiana Jones. It's escapist fantasy. He's the epitome of what all men strive to be. He's handsome, he's intelligent, he punches people, he's single, sleeps around, he's got cash, he punches people, he travels the world, he could sleep with any of his students, he uses a whip, he punches people, he goes on adventures, he can shoot people and get away with it, and he punches people. You get the point, he's not all that different from James Bond.
It's the appeal of the movies, not the character. Indy is the man's man from adventure serials, so when he's 73-years-old with a white head of hair (that he's swiftly losing BTW) and can barely keep up with the rest of the characters, you don't want to be him.
I'm happy Harrison is coming back, but to act like Indy can only ever be played by Ford is just fangirling. I want to see a fresh take on the character with an actor capable of stepping into Ford's shoes. If grandpa Ford has to carry the film and do all these crazy action stunts again, it'll turn out like Kingdom of the Crystal Skull.
I, going by indytim's guide, am a realist, if you couldn't tell already, and I understand if you love Ford as the character, but honestly, I think it's pretty ignorant and close-minded to be a "purist" and not think anyone can ever replace him. It's going to, and should, happen.