Sorry to add a spoiler to the subbed version of this finale Decade episode, but our theories of what was said between Kadoya Tsukasa and Kurenai Wataru was indeed correct.
Here's the exact word-by-word conversation between Tsukasa and Wataru from the finale episode
[HIDE]
Disclaimer: Excuse the parenthesis in the context of this conversation, the text written in the parenthesis is basically my personal commentary from the actual conversation being said
-Tsukasa leaves the Rider War World briefly and returns to the same night time scene from the first episode, then Kurenai Wataru suddenly appears-
Tsukasa: You are.....the one from before...
Wataru: Do you remember my words?
-Flashback to the scene from the first episode between Wataru and Tsukasa-
Wataru: You are the one who will destroy all Kamen Riders. Creation cannot start without destruction.
-Returns back to the current scene between Wataru & Tsukasa-
Wataru: You needed to destroy all Kamen Riders. Instead, you became comrades with them. That was... a big mistake. (oh snap)
Tsukasa: What do you mean?
Wataru: Now, my comrades (the real Heisei Riders minus Godai) and I will end your journey (OH SNAP!!)
-Wataru calls Kivat the III and henshins into the True Kamen Rider Kiva........(EPIC!!!!!)-
(I don't even have to say what happens next..........because it's obviously the Rider War)[/HIDE]
Basically what this conversation confirmed is that the Rider Worlds that Tsukasa saved where AU Worlds that disrupted the balance of the universe and the original Rider Worlds, which means the original Heisei Rider Worlds existed in the Decade universe.
It also confirms that all of the Heisei Riders in the Rider War are indeed the original Riders themselves (Shinji Kido, Takumi Inui, the original Hibiki-san, Souji Tendou, Ryotarou Nogami, as well the obvious Kurenai Wataru and Kenzaki Kazuma)........minus Godai Yuusuke as he was replaced by our own Yuusuke Onodera.
Pretty much, Wataru told Decade, "YOU FAILED, BITCH!"