Robert Downey Jr is the face of the Marvel Cinematic Universe and story-wise is the lynchpin of the Avengers. He will stop playing Ironman sooner or later, they'll keep making movies
Vin Diesel is probably the heart of this series but they'll keep making them when he's done too. The Fast & The Furious is what people want: high energy, high action popcorn flick that doesn't take itself seriously. Plus it has the market cornered as far as movies about cars go. Look at how Need For Speed flopped
It's not just the one man versus the world Liam Neeson-esque type of action movie that's made ad-nauseum (John Wick was dope though
). You can't write off James Bond, but this series will flourish regardless of who is in it.