Rage was shit, iD software had been coasting on crappy single player after doom 2. Quake 1 + 2 had shitty SP campaigns. Doom 3 they lucked out and made a just average game but nowhere near as good and fun as Serious Sam/Doom 2.
Rage could have been good but it was just so obvious people who made rage hadn't played many games in years. Developers who don't play a lot of games aren't going to learn the lessons other developers learned making first person shooters.
Rage's failure is exactly what iD software needed, let's all face this. iD software had been coasting for so long on their multiplayer and just being more technically proficient then other developers. Other developers began to surpass iD in terms of gameplay and iD never learned from these other devs.