Ganna isn't a cobbled specialist. And being good on cobbles is not the same as being a good bike handler, it's a different skill (though they do overlap). Even Nibali wouldn't be able to cope with the cornering of someone with a lot of experience on cobbles – only Pidcock can, really, and that comes from his MTB and off road background. All it takes is for MvdP to take a corner with slightly more intensity than Pogacar can cope with and it's a 3 metre gap that means with the added power van der Poel has, he's gone, as the draft is no longer there to aid Tadej. You underestimate how hard Roubaix is technically, and the difference this makes