I said in my review of The Last Man on Earth (1964) that any film version of Richard Matheson’s novel I Am Legend must must must have a commanding performer in the lead role, because that performer will be alone on screen for roughly half the movie. Charlton Heston is the epitome of the commanding performer, whatever you think of his acting ability — from what I’ve seen, he played one role extremely well, that of “being Charlton Heston” — so I suppose it makes perfect sense (or perfect Hollywood sense, anyway) to change the movie around to show him off best.

The Omega Man (1971)