Nice to hear that you are a supporter, or rather, defender for open-source. Also, like the term
I have always considered and illustrated the software development world containing 2 types of "camps" - open source advocates and "proprietary for profit" and I have always been drawn towards the open source, which also includes (although a bit off-topic) open to share ideas for the better good. I have actually founded a project via SourceForge in 2006 which was very close to cloud computing but as a Dropbox-like product, which became quickly popular among a small community (around 10-20 devs) who liked the idea but had difficulties to wrap their head around. Since the cloud and products for cloud file storage were just coming out, it's easy to see why this was a difficulty. Plus, it was a good example and a learning experience of my skills for promoting something that I know is a good idea but lacking the ability to explain it in a way to make it easier for understanding and seeing its potential. However, that skill has been improved but still far from being at an ideal level.
Sorry for going on that tangent, however I just wanted to share my passion for the open source community. That being said, I don't believe this project's founder(s) have any bad intent in the sense for being against open source. I admire the system they have in place for outside participation for being a part of its future plans, modifications, and updates.
Although, this is just my own thoughts and in fact have no clue if or what the end goal is, such as getting bought out, or basically for any profitable reason. But at the same time, I don't care to know as I have no place to speak on that just to name one reason. More importantly, that part of the topic as becoming open source or not, the point that I believe should be made is that it's not meant for this, for example a platform for other uses, very much similar to how this site is using discourse. The beauty I see in Slant is that its not really a platform that can be applied in other purposes (or at least not yet) but more something that needs to grow and is a platform in itself, which makes so unique. I can't imagine it being the same product in an open source way as in being used even with the slightest modifications.
The main point I believe should be emphasized in response to your last post
is that it's important to understand that Slant does welcome valid input for such purposes as you mentioned for why it could be a good idea to make it open source. Although, top users of meta slant do not have permission to contribute on a coding or bug fix level, I believe this is key to understanding the purpose for meta slant site and knowing why the restrictions exist such as viewing, editing, or anything related on the code. Simply said, we have no place for that. To finalize my point, I don't believe all code should be open for several reasons, but to name one that can relate to this discussion is so that the quality doesn't get affected.
I hope this was helpful to answer your question in a clear manner and also hope that I am overstepping my place as in talking about specific things in regards to the actual Slant team itself. If so, I apologize.