As many have said, Rika could have been in this group too, obviously because of her role in the movie. If she is not in it, I think that it's because Tsunku was serioues when he said that he wants GAM to be a full time group. With Rika in it, it'd just look like another special unit, and, moreover, Rika is already in a group (Let's remind that her graduation excuse was to focus on V-u-den). Rika in two groups of three nins would be weird. That's why I think Tsunku really meant his words and GAM will live on.
I also thought about the fact of Thanks! not being an Ayaya solo. Nakashima, Ito, and Otsuka all starred movies and released the theme by themselves, a solo single. So why would Tsunku lose this chance? He probably thought that Ayaya is popular enough already, and wanted to put Miki in there just for a) Miki will go solo again someday or b) Miki name in the press will be inevitably related to Momusu (meaning indirectly ads for Momusu).
Whatsoever is the
Why? of GAM being a duo, it so looks like Tsunku wants this to become a full time project. Actually, that's what he said. Can't wait to listen to their first tune
