Lack of innovation is the one I guess killing the forum. In addition with you example:
Consider Facebook in 2010/11 and now. Back in those days Facebook used to look like a simple php driven website and now it a massive application with enormous features.
Bad example: I liked Facebook more back then, before they forced a timeline and started deciding for me what I want to see (because they earn from it).
I've seen several forums "innovate", and in all cases I liked the old version better. For me, a forum doesn't have to change it's looks and behaviour once it works well and I know where to find stuff. A forum upgrade would also mean existing scrapers won't work anymore.
I do like the added Merit though, and I'd love to see more upgrades like it.