Not so much a game, more Codemasters dropping off a cliff. Without going too far back in to the nostalgic past, I think you can call that moment as being the release of Dirt 3 and the pick up of the F1 license.
Since then it's been downhill with rinse and repeat releases full of misleading PR, awful DLC practices, and poor support. The one glimmer of former glory was the DR kickstarter, engaging with the community and getting people back on board, which they've subsequently ****** up the wall by dragging DR2 in to the Codemasters garden of awfulness.. aka guff PR, DLC, and sketchy support.
They went from the very best to the worst.