First off, OP is correct imo about the fact that the forum is dying, or at the very least, seeing a definite stagnation in terms of content, as well as less amount of active users. Many older users have left this forum, either having been banned/self-banned or just become inactive.
Now that begs the question of whether something should be done? I think so, perhaps having more incel-related events, like @PLA1092 suggests, would be a fun start. I reckon that we should try implementing them first, and seeing if there is any "improvement" in terms of user engagement here and whether the forum still seems "dead" afterwards. But we are already stuck at this stage for a very long time because Master isn't active here much anymore and certainly won't be creating anything new for this forum.
If all that doesn't work, then only then should we even consider letting in non-incels and those who disagree with us. Even if it came to that, I am skeptical and still don't believe it to be a good idea because just look at ask an incel and what happened to it. So if we were to do something similar, it would have to be different to ask an incel.
Ergo, I reckon why the forum appears to be dying is due to a couple of factors, including: a lack of new updates and changes to the forum, many older users leaving or getting banned, and a stagnation in content with there being essentially the same threads posted every day.