Well, that's how I feel, I don't really think that applies to a general audience. I mean, if we can have Ichigo, Nigo, V3, Riderman, X, Amazon & Stronger without any explanation of where is Tachibana Tobei, what's so wrong with not having Gattack or IXA?
I really don't think they could have carried on the entire series like that. Look at the Black RX World. Original actor was there, with none of his supporting cast, besides mentions of a Kasumi no Joe who was never on screen . It had, by far, the blandest setting out of every world shown, and only worked because there was story development there (Dai-Shocker's introduction).
If the same had been done to other Riders, I can only see the Decade series suffering more. There's also the problem regarding the antagonists. They wouldn't be able to reuse previous antagonists without resorting to a "every previous monsters resurrects" plot every week, which would get old fast.
I'd have liked to see more of the originals. There was certainly enough space for it (the Nega World arc could have been used for it, and Last Story also could have used it, although I guess the problem there was length).
Alternatively, I guess visiting the original universes, even with cast deficiencies, could work if they had an original enemy faction alongside one new monster from each world.
That way, they'd have enough antagonists for the constant team ups, but each world could still keep some uniqueness. Although, I guess they didn't even consider an idea like that for Decade.
Toei had the first three actors in their press releases, and they were advertised as having returned for their roles, yet when the movie came, none of them did any voice acting. That's just unforgivable.
They participated in brief features for the marketing campaign of the movie (Stronger's and ZX's actors also were in at least one feature), but it was never explicitly said that they'd be voicing their original roles again... people just assumed that they would because it seemed like a logical conclusion. If they were involved in marketing it, getting them to record a few voice clips was logical. Although, apparently, Toei thought differently.