All I know is that CS has a cult following in all it's forms. There are still normal CS servers, just as I'm sure there are still CS 1.5 servers out there. (It was the golden age of CS after all). I'm sure everyone will find one that is most to their particular taste. I'm not ultra excited about GO. I'll be fine to just casually play CSS when it comes out, I expect.
Great communities make CS and this place seems to have found that happy medium between silliness and seriousness. It's not a total madhouse of chaotic children, and it's not a bunch of lifer "Pro-Gamers" who rage all the time about how everyone isn't as talented and as obsessed with the game as they are.
It's kinda like a medium security nuthouse with a few hilariously fucked up people and no real genuine psychopaths. (Or, if there are genuine rage machine psychos, they're well-medicated.)
IS BESSS BRRRRRRRRRRRO!
I'm sure there is a certain demographic which will get GO, some of which will love it, and some of which will go back to Source.
I doubt it will be a massive Exodus, and ultimately, I don't think it kill the Source Community on Darkly, and to me, that's all that really matters.