I wonder what led to the decline of activity.
The stats are indeed quite sad to watch and now that many of the original people left, it might have put even more people off. We're at about 10% of the usual activity now, which is quite scary.
But Discord isn't and will never be a viable replacement for a forum. It's also, as already mentioned by gameblabla, a commerical service, with no gurantees in long-term availability and there's no control over the code.
Hopefully CW will recover again. Omni seems to be completely dead by now, considering that you can often see messages that are 1 day or older in OmnomIRC.
And Cemetech does not appear to be all that active anymore, even though it's still a lot busier.