the whole DB/DBZ/DBGT thing is done in Japan, afterall, DB started there in like 1989. DBZ started airing in the early 90's and DBGT was over in about 1998...thats nearly 5 years ago, so the popularity is like..nothing. Why do you think American developers are the ones developing the new DBZ games?
I think the series hasnt eneded in the US yet, although I wouldnt know, as I dont have Cartoon Network...
I think after DBGT, the whole DragonBall idea will be finished, its getting rather boring now...well, I stopped watching at when Buu was first revealed...the Frieza and Saiyan saga were much better IMO.