I don't really think you can expect people to put the same level of commitment into something that doesn't change. The person gets older with the game, thus without new people coming through the community was always going to slow and stumble. That was caused by the slow development. If you look at the top of irental, you will see a lot of the folk that played LFS, and who have now moved over there... why? Because its new, its constantly new and it has a challenge, whether it be the racers or the new content.
It's not about the Sim being perfect (its not, or wasn't I don't know anymore but LFS has always been far from perfect) but irental produced content change after content change, giving people an opportunity to drive many different combinations of track/cars/class... full servers after a proper advertising campaign... for those that moved over, that was what they had on LFS, but eventually the same stuff gets... samey.
At the same time Live For Speed slowed (and nor can you blame a team of 3 for slowing, their baby has been a great success, but imagine the burnout!)... you can't blame people for getting fed up of an old game, that is the nature of gaming, you move on. Yes the classics still feel fun, but eventually you get to the same place you put them on the shelf in the first place...
What actually happened is the community came out of its initial shell... plenty of the older people still sim, and even still sim together (ffs Jack Sedgwick races for ineX at times...jaysus upside down etc!) so the community spirit fostered at LFS is still there, but they moved onto a fresh shell, with more stuff to do, more quicker people, more events... isn't that why people game online... challenge?