It's been awhile since I saw the numbers, but Ryuki wasn't that huge of a step down from Agito (which is, I believe, the highest rated Heisei series). Fourze, however, is apparently pulling in worse numbers than Blade and Hibiki (although those two also had big toy sales problems, whereas Fourze doesn't).

You think the decline has to do with the shows, or has Kamen Rider run it's course?

It's just ratings.
Sentai has had plenty of bombs in it's run and it's been going on strong.
Kids in Japan just don't have the interest in Fourze.

Pretty much, yeah. Honestly, Superhero Time as a whole isn't doing that great - GoBusters is apparently tanking hard. Part of that may be it being dragged down by Fourze, but Sentai in general has been declining in popularity for awhile now. If either of the franchises are in danger it's Sentai, but I doubt that will go anywhere so long as Power Rangers is still doing well (although PR does have a buffer zone now that it's three years behind and running each show on a two-year cycle).

I am curious as to some of the reasons why Fourze would be doing poorly, ratings wise. I'm quite enjoying it, myself.
Either way, it's not like one series getting poor ratings will kill a 40 year old franchise. It would take multiple :lol
(And I'm pretty sure Double and OOO did well, ratings wise).

The tastes of Japanese audiences have always been hard to decipher. I have given up trying to predict what Japanese audiences like and don't like. Apparently Toei is having trouble too. What do you guys think?

I'm thinking it's the tone of the series. While W and OOO had plenty of comedic moments, they were still fairly serious series.
Fourze is like Den-O on crack (At least Den-O had the Yuuto (Not Zeronos)/Airi thing for drama), with only...4 notable dramatic moments (Correct me if I'm wrong, but The opening, Gen's "Death", Ryusei's redemption, and Eleen's whole story arc).

Does it now? :P

I can't imagine any scenario where Toei would think that Fourze going for another year is the thing to do when they've nearly exhausted the toyline and it's already become easier than in years past to get every collectible trinket in some variation, to say nothing of the cast or the storyline continuing beyond September. Fans not seeing the 2012-13 Rider and Toei not doing one are not the same thing.

I think the reason Fourze isn't working well at all is because it's too ... I don't know, "anime-centralized" ... I feel like I'm watching a show that's trying to be a live-action anime at times. And let's face it ... as much as I love Fourze, that suit is just not Kamen Rider coolness, it's goofy looking, the show is more goofy than serious, and for a casual Japanese viewer that's not going to translate. It's like making fun of itself in a sense?

I love it ... but I love the cheese, we don't get enough here in the States.

I'm sure the Movie Taisen this year would touch on their graduation.