Every F1 WDC that lived/survived, except for Hawthorne, Stewart, Prost who retired as active WDCs and Brabham and Fangio to a lesser degree, were fallen/ zeroes.
Farina, Phill Hill, John Surtees, G. Hill, D. Hulme, E. Fitipaldi, James Hunt, N. Lauda, M. Andretti, A. Jones, N. Piquet Snr., N. Mansell, D. Hill, Häkkinen, Räikkönen.
"I'd also disagree with that statement. It's one thing to go on after you should have retired as was the case perhaps for Graham Hill, Alan Jones and Nigel Mansell but another to still be winning races in your final season before calling it a day.
In the two years after his final championship, Mika Hakkinen won 6 more races and after two awfull years in a declining Lotus team Nelson Piquet came back to win 3 races in two years with Beneton. Hardly a sign that eitehr driver had declined and fell." - Cider and Toast
Also Raikkonen, won a race in a car that hadn't been developed for half of the season, and apparently worked miracles with that car according to his engineers. I would like to think that he was still at the top of his game