I'm surprised that he's so bad in the Ardennes but i agree with Koronin that such a decline can't be only for the age and i'm still convinced the age related decline is only the one we saw last year in the Vuelta in the only two real mountain stages at the end of the race, probably more evident this year but limited at real mountains. In Sanremo and Ronde he showed that he was there but also on Jabel Hafeet at UAE Tour he put on a strong showing and in Murcia was very good so i think that motivation could really be a factor.
Looking at World Champions reminds me what happened to Cipollini, after a stellar 2002 ended with the WC he suddenly became a shadow of his former self (IIRC he won only two stages at the Giro) but in his beloved Sanremo he showed that he had the legs riding in front all of the Poggio and then easily winning the sprint behind the attackers (look at the faces in this photo he recently posted, he was joking with the competition
https://www.instagram.com/p/Bscq1lbl_Km/), i've always thougth that a performance like that wasn't possible for someone phisically done and a so sudden decline without other issues is unexplicable, the same thing that now comes to my mind looking at what's happening to Valverde.
Anyway i don't believe the rainbow jersey is cursed, i think there is a mix of factors that probably hamper some of the wearer of that jersey starting from a busy winter that can derail the preparation, but you are also the most controlled, some lost motivations, some feels the need to demonstrate they deserve that jersey and made mistakes that cost races, some feels the pressure and god knows what else.
But we still have a lot of cases of great seasons from World Champions, recently Sagan changed his charateristics putting on weight after winning the first one but he continued to win a lot (including his only two monuments) and he's crap this year after losing the jersey, Cavendish was pretty much the same, Hushovd in 2011 looked stronger than ever and in the Tour he was in yellow for a week surviving even the MTF in Super Besse in the 20 men GC riders group and then he won two mountain stages from breaks beating riders on paper stronger than him like Moncutie and Hesjedal, Evans was the same of previous years, Bettini the same, Boonen had his best season in term of wins with the Ronde among them and we can go on in the past with GTs and Monuments winners as World Champions like Lemond winning the Tour, Argentin Liege and Lombardia, Hinault Roubaix and Tour, Moser Roubaix and Lombardia, not to mention Merckx that as World Champion has won two Giros, a Tour, two Sanremo, two Liege, a Roubaix, a Ronde, a Lombardia along dozens of other races. And maybe i'm forgetting someone else.
It's the fourth time i try to post this message because i was pulled out...