In my opinion Decade hasn't been retconned, just this is the 'World of Kamen Rider' where all the series happened rather than any of the ones where it's just one alternate series. Heck by my view this world wouldn't exist if it weren't for Decade. (I feel it's the remains of the Rider War World with some aspects of both the originals and ARs if you like, hence why we have Kiva using the Zanvat Sword, N-Gamio-Zeda, Llumu Qhimil and probably some other stuff, while also having hints of unseen worlds as well (The Hopper's Bike)) But thats just my view.