If you would like to see them add Brainstorm to the new architecture, you can file a feature request: Feature Request/Bug Report Form
In the meantime, you can still use Brainstorm. You can download and install CS6, CC, and CC 2014 as well as CC 2015 from the Creative Cloud app. All of those versions are included in your subscription.
I can't say that I miss it.
It wouldn't bother me if it was brought back, but the button for it needs to be moved far away from anything else that might need to be clicked.
I spend a lot of time in the graph editor and I can't tell you how many times I've accidentally clicked brainstorm instead of the graph editor.
The button was moved in a recent version (I think one of the CC 2014 versions, but I don't remember offhand). It was a very little-used feature, so the AE team decided it wasn't one of the things that would be included in the new architecture (at least, not right away). Other things took priority.