I honestly can't remember any other former F1 WDC unraveling in the way that Sebastien Vettel has. Several had ups and downs after winning the title - especially those who won based on good driving mixed with more than their fair share of lucky breaks. What I mean is, some of those guys weren't incredibly brilliant to begin with (no offense to Damon Hill fans
), so yeah, they dropped off some. But as far as just falling apart, nah. IMO, Vettel began to show cracks when Daniel Ricciardo started showing his stuff at Red Bull... and Vettel exited stage left for Ferrari the next year. Now he's got this Leclerc kid to deal with, who I think is even faster than Ricciardo.
In years to come, after Lewis and Vettel are gone, I expect that it'll be Leclerc vs. Verstappen for the WDC. Course, Kimi will still be out there. He'll be in his late 50s, but he'll still be out there.