Harvey crossed a line, that's why he's toast
The casting couch thing is real, this is pretty standard quid pro quo thing in Hollywood. But notice that Harvey was the only real Hollywood big wig that's in trouble? The fact that Harvey Weinstein was the only one called out about sexual assault in Hollywood leaves me to believe, Harvey was just the one that stepped over the line with his aggressive harassment.
There's others too like Bret Rattner, but he doesn't have nowhere near the amount of accusations thrown at him. There's noway he's the only studio head with that type of body count. He just got singled out for being a overzealous creep where as other studio execs wasn't as off putting and predatory/aggressive with their propositions as Harvey was.