TLDR; adding the ability to limit student users to interact with users in their own sections to graded discussions will simplify the setup and allow course designers to make the course more clear and efficient for students, teachers, and designers.
For high-enrollment (200+) multi-section courses, there seem two ways to create smaller (~30 students) class graded discussions:
1) Use sections and create the same discussion for each section which bloats the grade book and the modules page,
2) use groups which creates a whole new course area giving students access to tools we might otherwise limit, disallowing our ability to limit students' ability to create discussion topics (which can cause confusion for other students), requiring a multi-click back-out process for instructors who potentially need to see and participate in each group's discussion.
Option 2 requires the additional use of sections to limit grade book views for grading within the sections and makes it difficult for a course to use the groups tool for what is actually intended: small group collaboration.
Differentiating Assignments is not effective because students can still see the posts of other users even when they do not share a section.