I'm new to Dirt Rally (long time RBR enthusiast), but I've quickly been impressed by the basic game. It has a lot of potential for an extended life. The one thing I can imagine that might cause a decline in participation could be the current set of tracks. While the current set of tracks is quite extensive (for me not an issue now since I'm new), I'm sure the release of new tracks across the WRC world would be a huge boost. For example, it would be great to see Australia, New Zealand, Mexico, Argentina, Spain etc. added at some point.
New track packs (perhaps supported by car packs if necessary) to keep it up to date with the FIA WRC would keep the game alive, even if that means releasing expansion packs at modest prices to cover the development costs (eg. similar to Assetto Corsa, RRE).