It's a simplistic way to view things imo. As I mentioned elsewhere it was possible that we could have maintained some level of success post-Bennett, there were succession plans in place and the core of the team was stable - it just didn't have any luck and Price was not up to it. At Newcastle, again I personally doubt Bennett deserves as much blame that he gets there - the Nathan Tinkler situation was a far greater influence on their destruction than anything he did. He still got them to a prelim final in his 2nd year, and may have gone further in year 3 had the Tinkler implosion not occurred.
Also worth pointing out that the Broncos went further in 2009 with Henjak than they did in 2008 with Bennett. Also the Raiders did pretty well after he left Canberra in 1988.
Perhaps Bennett, being one of the greatest coaches in history, is simply a tough act to follow. There doesn't need to be anything sinister about that - he goes all out to win. There isn't much to compare him to - who is at his level in recent times that has changed clubs? Bellamy and Trent Robinson haven't. Hasler maybe? He's had a similar effect upon leaving Canterbury, but the Eagles did make a grand final a couple years after he left.
Perhaps Jack Gibson, generations ago, is the only possible comparison?
At Easts, Gibson won the comp in 74 & 75, prelim semi in 76. In 77 he left and they finished out of the semis in 78 and 79, until another supercoach, Bob Fulton took over.
He won 3 straight premierships with Parra in the early 80's before handing over to John Monie who won another in 86, before the club fell off a cliff in 87.
Then at Cronulla in the late 80's they really did nothing, before actually improving after he left. This gig was more about building for the future than gaining instant success.