Ahh the "I have a life" argument. Classic.
Heres what you don't realize, just another perspective. Yes YOU don't care about the achievement, YOU don't play online. Theres enough of you as well to be a vocal group (in the 5% of all 5million+ GT5 players who are posters on this forum).
What YOU and others don't realize is just because it doesn't matter to YOU doesn't make it any less fun/important to other people. The fact of the matter is the designers made a game they intended to be played a certain way over a long period of time. People buy games to play for a challenge. Yes not everyone does, as evidenced by the fact that you an many others like you are very vocal about wanting to just be able to drive around and tune cars.
The problems being the following: (and lets ignore that yes it is not perfect) the game is designed to be played over the long term. The fact that credits are limited means they also designed it so that theres so many cars theres variation in races as everyone can't easily own every car. They designed a level system so that at a glance someone could see whether another online player is experienced or not, as well to allow the unlocking of certain events. The intention was for people to work up to certain cars and certain races, and play like this.
However then cheating comes into the mix, and everyones duping and getting every car, and you have low level players with cars like the X1, etc etc. Now rare cars aren't rare, level is just an arbitrary number. I bet their original hope was (for example) high level players could race eachother in their X1s online easily in a open lobby which is now completely impossible.
So yeah YOUR gameplay isn't affecting others but the cheats and culture of cheating IS affecting people who don't - outside of our little microcosm here -, and so for the greater good these things need to be stopped. The game isn't about YOU its about everyone, and who are you to decide whether or not its ok for someone to enjoy the achievement in beating races and leveling up, or putting the time in at higher levels enjoying the cars they have, marveling at ones they want, and eventually saving up to get them. You don't HAVE TO cheat, or HAVE TO grind, you want to, because you want every car. Not the games "fault" or other players, just your own.