Hmm I don't think Tonne is being prepped as a replacement. I think Tonne, a stylist, taking over the magazine would be a radical change for them. It's gonna be US Vogue turning into VP where the EIC is very hands on with the fashion features, making it more fashion oriented. We all know US Vogue is not just a fashion magazine but also dives into political, economic and social realms. But I see Tonne being groomed to be the next Grace Coddington of the magazine.
Elle, Vogue and Bazaar has followed this formula for quite some time now and I don't think they're bound to change it anytime soon. I think it's what makes these magazines work - the lack of direct participation of the EICs in the features thus making every issue a collective effort / vision of the entire team and not just of the EIC. Look at what happened to VP, the EIC being hands on with the fashion and her vision being 70s/80s oriented... Thus everything shifted that way.