I've noticed that posting to certain underutilized forums is not successful. I've started to look at the dates of posts in order to decide where to write my questions.
Perhaps some forums should be consolidated by function rather than manufacturer. I know that might be impossible, if you have relationships with vendors/advertisers. But it would be better for your forum communities.
Specifically, I've been looking for info on external video recorders, and have resorted to one manufacturer's forum to get advice on all manufacturers' products.
We prefer the term "evolving" but yes, it has happened a few times along the way.
It's tricky, because forums start as places to solve problems, but over time, problems become less frequent, making critical mass more difficult.
The kind of consolidation you mention has worked for us before, for sure. We're doing it for most of our storage forums right now, for example, for this very reason. Tell me more about some of the other ones you think might benefit from this approach.