At the end of the day, all I can say is this... Decade left me with a feeling that Decade himself will always be traveling around worlds ready to bring together all the Kamen Riders if it is ever needed. They all might be alternate versions, but if luck sees fit, he might be able to find his way into an original/actual rider universe; which is why he and Double were able to fight together two times. So if you care for only seeing the Riders and the powers they weild but you honestly don't care about seeing actors or the stories they may have, then Decade can give you what you want. So, in his own way, Decade really can bring all the Riders together. Just don't expect them to be exactly the same as you remember them.