Time will tell, but I think pure statistics (yeah, I'm suddenly descovering my mathematical side here) point to two of them becoming real legends at best. Well, depends on how tightly you define legend of course.
My list of cycling legends, containing some riders I have never seen live:
Coppi
Merckx
Anquetil
De Vlaeminck
Hinault
LeMond
Indurain
Pantani
Contador
Boonen
Froome
Sagan
In the end a "legend" is not just the sum of his wins I think, but also his reputation, his story; and often a legend is not just made from what the person themselves does, but how other people treat them, what they think about them, what they write about them, how they are inspired. There's a reason for the word "legend building". The narrative of a legend is at least as important as the achievements. Someone not so big can become a bigger legend with time. Someone with incredible achievements doesn't necessarily become a legend, and if he's forgotten there is no possibility to say "he's a legend, he's just forgotten", because legends are simply not the people who are forgotten, by definition I think.
And has Marilyn Monroe really done something so extra-ordinary in her life? But I don't think there's any doubt she's a legend. A legend is a kind of myth, in the end. Hence I'm leaving out Sean Kelly, Cancellara and Gilbert (although with Cancellara I'm really on the edge here.) Hence Boonen is in although I seriously dislike him. Froome is in despite that many people will call him boring and a product, but he shaped a decade with his GT dominance and his Ventoux run I think is some serious legend stuff. Hence LeMond is in. Hence Valverde isn't, although he might be at one point simply for his longevity.
Oh, and a "legend" cannot be someone who's mostly remembered as the biggest cheater, sorry Lance...