/collab/ isn't dead. This amount of activity looks pretty normal for an off season to me, but you're the one with the stats.
There were always talk about board changes when I was a tad more active but somehow, every few month when I returned to take a look, /collab/ always stood there, timeless and unchanged. As timeless as a place that's only been there for 5 years anyways. Mods change, admins change, other boards are created and disappear, whole new imageboards are made and merged, and attempted takeovers happen, but /collab/ is always /collab/. I wouldn't really even chime in if I didn't see /dis/ (the other slow board, a.k.a. board I can actually keep up with) bite the dust. I'd hate to see someone else who also checks in every 6 months to come back and see /collab/ isn't there anymore. Its a great place to browse back once every few months to see what slow churning projects made progress.
Although it looks like the fandom as a whole is dwindling (a short Season 3 and long waits between any two Seasons after that). Some of the frequent updaters stopped (misterbull, supersayanmikato, the Harvest Moon group, etc). Even Markusmaximus stopped. Also, later nhydock stopped. Though we are sure to lose a few once in a while, it does seem like they all vanished around the same time. So to fix the first sentence of this post, I'd say /collab/ is dwindling more slowly than anywhere else.
On the other hoof, I'd be curious to know what would have happened if the three boards were one from the start, but it moved at /collab/'s pace. (When active, /art/ can be extremely fast in comparison, so much so that artists were kept to their own thread instead of making one for every new project/piece. I think /fic/ is also faster in active times but mostly due to discussions.) I feel like each group would mostly end up with people with very similar skills, except for those with nothing "declared" in advance. There'd be a few exceptions, of course, but it seems like in some cases, viewpoints are significantly different (about what a project is and how to go about it). They'd get to see the results from eachother's groups but