Anyway, I think the premise of this topic is quite wrong. Decade only was really extremely overpowered with Fury Form, and that form only existed when he accepted a mission that would result in his own death.
Otherwise, yes, he was still powerful compared to most other Riders, but so was the powered up Skyrider (well, at least, in his debut), so was RX, so was Kabuto, so was Double in Decade's movies... It's not like this is a new concept.
Decade is powerful for quite literally no reason. The story isn't made better because he's so strong, in fact it's frequently made worse. People don't object to Decade's powers so much as how his powers don't make the story any better, though.
I really don't see how you can say that.
Although it wasn't an active plot element during most of its run, Decade was introduced and often described as the one that could defeat Kamen Riders. The entire set up of the plot (which wasn't actually the focus of the show...) was based on that idea. If he weren't the "destroyer", the series would need a completely different premise. I'm not saying it couldn't be done, it could be done, but would be a different series.
The world hopping element of the show, which was the actual main focus, didn't need such a powerful Rider, but removing it would still change the series significantly overall, so I don't see how you can say that he's powerful "for no reason".
If he can turn into Kabuto and Kuuga, why the hell does he bother ever turning into anyone else?
Kuuga's basic forms aren't that good. The only one of Decade's forms that really had no excuse for the little use was Kabuto, but shortly after he got Complete Form, which was only
touched by Super Apollogeist, so it's not like he lacked power anyway.