Yes, there just aren't enough people here anymore to sustain conversation in both places. I do get the problem with Discord in that regard, people have made various points but it was too long ago for me to now reply.
However, I don't think we are the only community that has ended up moving to Discord. I think the larger ones can have conversations going in both places, but for us it just isn't possible.
Not sure how popular this could be, but we could try shutting down Discord for text chat (opening it back up for E3 conferences, of course!) and give things a go here. Ironically, some of the features people felt were lacking with Discourse initially are now in here, such as being able to see who is online, who is writing a reply and user-selectable themes.
Or we could just accept that this community is not getting any larger, and it seems that we as individuals seem to be getting busier in life too with diverging tastes in games, which certainly doesn't help. I've always felt that, despite all the debates about forum software, it is the community itself that is more important, wherever that exists and Discord is a convenient way for those of us that remain to keep in touch.
As for things being quiet on Discord, I have had a crazily busy few months at work and on top of that I am looking to move at the end of the month. Hopefully by the end of summer I'll be back to playing (and talking about) games - wherever that may be!