Forums are a declining medium and have been getting replaced by social media for years now. New forum development has been effectively killed and those that predate most social media are gasping for air.
Fansites themselves are doing a bit better but not much. I wouldn't say the RS Wiki is to blame. It's more the lack of new players. Experienced players don't need help as much as new ones. Also fansites support would help but it's not the major issue. Fansites and Jagex basically ignored each other until 2009ish and everything went fine. Though I agree that Jagex is giving clear preference to reddit by posting there constantly while ignoring older sites such as tip.it.
Although defeatism doesn't help the cause, the health of the fansite and the forums are largely out of the hands of tip.it and other fansites at the moment.