Most online communities go downhill as they get larger and become diluted, the core founding posters simply move on or are drowned by newer users.
Users experience the grass is greener back in the good old days thought pattern that all people are prone to, nothing is ever as good the second time around, pioneering is fun.
I don't think that is the issue here though.
There is a structural issue with the layout of the forum, most forums have a general category (for broad discussion of the forum's core topic), targeted sub categories, and an off-topic category.
Here we have the general category and off-topic category rolled into the one sub-forum. This isn't a big problem when the community is small, but as it has grown, and investment opportunities have declined, it starts to drown out on-topic discussion.