I can't be bothered trawling through the posts to work out who said what, so here's my general rebuttal.
First of all, the most successful and popular flight sim of all time is JUST flying a plane. No combat, no bombs or guns, no other players, just you and the physics of actually flying a plane.
MMOs are a unique animal in that you can't actually win at them and it's rare that top-end events are quest-related (outside of progressing as a guild rather than an individual). Most end-game content is repeating the same raid instances over and over until you've all got the best gear. Achievements have been added to some to make the concept of farming more interesting. Which I think says a lot about the game content.
If you can't win races or practice very often, then what difference is having a line of text achievement going to make to you? You'll know when you've covered distance or something? As I said before, LFSW will tell you that and you can already chart your personal improvements. It's no use for showing people who don't play LFS, because they don't care. And people in LFS don't care because they would already have them.
It would negatively affect performance, since it would be adding more load to your CPU and the LFSW server. And since there's already a million physics calcs slowing people down, more calcs would be a stupid idea.
And it doesn't add to gameplay because any achievements added would be things you're supposed to be doing anyway. Maybe punishing people for NOT doing these things would be better? You overheat your clutch 10 times and it bans you for 24 hours.
If you find LFS boring as it is, then clearly sim racing is not for you. The clue is in the genre name. "Sim" and in "simulation". And "racing" as in "the desire to finish in a high position by moving quickly". That's it.
Achievements and trophies work in console games because they have been built from the ground up to be far more suited to people playing for shorter periods. They have the networking in place to track everything the player does with the express intention of rewarding them when they accomplish something the developer thinks will keep them playing.
And that's all it is - false replay value. Because it's there and incomplete, you feel obliged to go and fill that bar. 15 or 20 years ago games had replay value because you had a good playing experience, not because the instruction manual said there was a secret and if you repeated the same task 400 times you'd see a secret ending.
The comparisons with Forza are a bit unfair. It's geared as a mainly single player game with a multiplayer aspect, so it needs cups and challenges. LFS is sold as an online racing sim, so everything in there should be player-generated. There are loads of really professionally run leagues and cups running, so just imagine those are a virtual experience.
It's not about liking or disliking an idea, it's about what it brings to LFS and the community. I'm of the opinion (although it sounds slightly snobby) that if you don't want to get online and race as fast as you can, then online racing is probably not for you. It makes sense. I don't go out and play football because I'm not interested; I wouldn't join a club and then try to convince them that by changing the rules and adding some guy to track the distance I run per match, I'd suddenly be interested in playing football.