I like your definition and that does clarify things - then all my design conversations are theory conversations. I think most people think of theory as somehow more abstract than that - and would be happy to know that its not. So to answer the post's original question, design conversations are doing fine - wherever they happen. It's just they're usually focused conversations about systems - and rarely get more abstract than that. Does that make sense?
My understanding is that in the Forge, there was a lot more High Theory - more Theoretical Physics than Aerospace Engineering. I don't think I would've enjoyed those conversations very much.
(And yeah, the historic split in "system design" or "adventure sign" seems irrelevant to modern indie games. But the difference between invisible and visible rulebooks is still relevant - analogous to system and "play culture".)