I honestly don't think this is 100% true. There's a lot of discussion in the Redditalternatives subbreddit where users are seeking other avenues and looking for other alternatives to Reddit. It's a good section to advertise your forum as well. I'd recommend being active in that particular area. There's also various topics in that sub where users discuss forums as well. I believe it's a quality avenue to drive in new users to online communities.People will stay with Reddit even when it starts to sink into the dark deep. Just like Ravenfreak said, people are lazy and if they even have to click one extra times, they'll drop it without a single thought.
A lot of people thought Digg wouldn't go anywhere years ago, but we all saw what happened to their community years ago. A lot of their users ended up flocking to Reddit. There are many things that are going on behind the scenes on Reddit that their user base aren't agreeing with, so it's a good chance that they'll leave for a better community.