Given that we still have daily posts of full drafts to the Ideas forums and people posting drafts without greenlights, having two forums would likely just generate a ton of extra work for mods who would need to move threads to the correct forum.
Also, there's no guarantee that the Ideas forum will actually be used for discussing ideas rather than becoming a dumping ground for people using the shotgun method.
It seems like the needs of people seeking a greenlight and more experienced authors seeking critique are very different and it would be sensible to have different forums to serve their needs.
This might come off as elitism. Also, what if a first-time author doesn't want greenlights, and just wants to post a thread with the seemingly designated Cool Kids? Greenlights are only required if someone wants to make a thread in the drafts forum.
Also, I feel like experienced authors shouldn't need an entirely separate forum to themselves for conceptual discussion. If it's an issue of attracting the attention of reviewers, they can always reach out to someone via PM.
ETA: another thing… it's pretty rare for someone to receive greenlights without needing to make at least some tweaks to a concept. A lot of authors are "just looking for a greenlight" but it's rarely a case of rubber-stamping at a glance and more like getting an abstract repeatedly revised and approved by supervisors before a paper can be written.
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