I been playing racing games since Pitstop on the Atari. I know all the real world tracks like the back of my hand, and I'm not into cars. Never got into tuning beyond some gear ratio changes and ailerons (I guess spoilers for cars) to increase top speed. Never really cared about all the upgrade components which just meant more work to swap them out to meet the correct performance rating.
Plus what do car upgrades have to do with career mode? A proper career mode would be fine, upgrading the car however always felt like cheating and breaking the game. GT did teach me about the different components in cars, not that I can now find the air filter in my own family car though. I did change the battery a while ago, dropped a wrench in the bottom of the engine housing which was a bitch to fish out. For everything else, the dashboard tells me when to bring it in for what maintenance. It won't be long until cars will drive themselves to the garage at night for servicing :)
What it needs is offline saves. I just lost almost 2 hours of progress due to server maintenance and refusing to reconnect after the server came back.