To be very honest, I think this is too premature. I agree that GA has said she will not participate in a 12th season. But this does not exclude her participation in more singular events like (TV-)movies, or single episodes, imo. CC and DD also never openly said that the X-Files were dead. Quite the opposite is true, imo. So, when it comes to the series itself, I see not enough evidence for terminating this board. Just imagine an XF event happening in couple of years, and we're closed....
Now, when it comes to how many posters frequent this board, this is a different question. I know the FF rules, and I understand that this board needs to close if we don't bring in the required numbers. I can only hope the numbers stay the way they are. I know it's a burden for everyone, since one feels like having to post a lot, just to keep the board open, and one doesn't always have the time for that. But if you ask me, we have no choice. We have to keep going.