Full disclosure: I'm the current maintainer of KallistiOS. I don't think that really biases the rest of the post, but I figured I'd mention it, since it is at least on the topic thereof.
I don't think there'd be any sort of homebrew renaissance if Katana were to be released (not that Sega ever would, since they don't have the rights to release a lot of it even if they wanted to -- a large part of Sega's official SDK wasn't written by them, after all). Why would doing so attract the large swath of new developers that would be required to have such a thing happen, after all? Also, I'm reasonably sure that GD-ROM production is gone forever -- there will never be any more pressed GD-ROM discs made by Sega.
Also, Sega's tools aren't necessarily easy to use either. A small development team is better served using KallistiOS than they are using Katana -- there's more people around to support it after all.
Finally, it is just as possible (if not more so at this point) to create an awesome title using the homebrew tools available as it is with Sega's tools. I think that many people in this thread seem to think that just because people haven't made massive 3D homebrew games means that KallistiOS doesn't support 3D to any major degree. I assure you that is totally incorrect. Recently we've had a bunch of talented people (PH3NOM, amongst others) working on producing a much more sensible OpenGL-like implementation for KOS. That OpenGL-like library makes 3D homebrew even easier to make, especially for anyone who's worked with OpenGL in the past (which is a whole lot more people than those who have worked with Sega's libraries in Katana).
Simply put, Sega's never going to release Katana free of charge, and certainly not as open-source software. Even if they would do so, I don't think you'd see a major renaissance of homebrew anyway -- there's not enough developers that'd be interested in doing that at this point. Yes, a lot of people around here are very keenly interested in the Dreamcast, but in the rest of the outside world, not so much.