She'll be back, just not as much.
You know, Moffat's words are starting to ring true.
It's hard to explain but not to grasp River Songs timeline.
Last season it was all about River. We saw her birth, and that's when we figured out her secret. That was about the time when the older River faded from the episodes, and the young River just begun; her rebellion, her growth, her change.
So most of the last seasons episodes deals with the younger River Song, not the one we're used to seeing in Amy's first season (an older River Song).
And also, the older River Songs development, the closer she meets her end.
I'm not sure if everyone quite grasps that her timeline ends with the 10th Doctor at The Silent Library.
So we all technically know when she dies and how.
It's all just a matter of plot placement, and not getting her too involved since we basically already know 98.8% of what we need to know about her, if not 100%.