[Suzu]

Actually, Decade did the same thing as Gokaiger though. The world's Decade was visiting where A.R. worlds, while the real riders did seem to coexist in another world- or that's what it suggested. But then again, Decade was like: Screw logic! We'll just do whatever we want in every episode.

The thing is, the shows are presented seperately for every generation to enjoy without knowing all of them. It is easier to build a superhero show with a new team without some overpowered senpai appearing. Of course one doesn't have to like the crossovers or what they did with the continuity in Gokaiger, but I try to use my own imagination on how Gokaiger works. Headcanon can help you get over such things, even though it doesn't correct the plotholes, but just covers them up.