250s and 125s have been faster for a long time how, even in the '70s. The thing that makes bigger bikes attractive is the fun factor. They're beasts that demand respect. If you get on the beast and respect the beast, you'll have a ball. If you hop onto it the way a lot of these clowns get onto a 450F, with little riding experience and even less common sense, you'll get your ass in a sling.
Also, a 450F comes with a lot of things that make it, in my opinion, more dangerous than a 500. It's heavy as hell, for one thing, and the engine braking makes it easy for a novice to jump into the air and land on his neck. Notice that no one even thought of the Leatt brace in the age of the 500, but it suddenly became a must-have item in the age of the 450F. They're big and capable machines, but what makes them dangerous is the fear factor. They don't have it. When you get on a 500, you can ride it about and have a good time, but then when you go to screw it on, it'll scare you so bad that you'll treat it with kid gloves until you progress to a level where you can use the craziness. A 450F just says, "Hi kids, I'm cocai... I mean, I'm a dirt bike! Let's go fast!"