I'm new here, been reading a few months, so it can't be that bad. I like it actually, lots of useful info, but it can take some digging to find it.
I've used a lot of forums over the years, though reasonably new to running, hence new to the running forums.
I think this...
"and the remainder settled into long running threads which didn't encourage new blood."
...is a key observation. I've not seen a forum with quite that approach before, it's damn near impossible to get up to speed on anything, because you might have 20,000 posts over several years to read through in a single thread to understand what is going on. No small task.
I don't know if it was a deliberate effort to make the forum less cluttered or what, but its kind of intimidating for a newb. It's a "you're ten years late for the party" feeling. Especially as it seems quite a close community here, from what I can gather. Lots of you seem to know each other on the outside.
Just my two pence.