I don't think that's quite right. If the actors were available easily, they could have brought them back in secondary roles without overshadowing Decade. They did bring back several of them without overshadowing him, after all. They just would stay in short secondary roles, not taking the spotlight.
That said, I guess they probably could have featured more original actors, but it'd just hurt the consistency of the show. There's a reason they made Decade's Wataru a kid, even though Kiva had just finished airing. They wanted to make it obvious to the audience that those words were AUs, they weren't trying to hide that.
I disagree. The 9 initial worlds generally WERE clearly alternate takes on the previous series, besides maybe Ryuuki's, although at least it still kept the focus on a Rider battle. It's not like they were generic ideas that could have easily fit in with any costume.
The concept of alternate takes on stories aren't slaps on anyone's face. So, why would Decade's AUs be an offense?
I can understand complaining about Diend's summons. They were just a stupid concept meant to increase the fanservice, but complaining about AUs in general makes no sense.