I only asked about LMU because it seemed like from your comments that it might be a recent struggle to find enough popular races, and LMU is the only thing that has released recently that I know of.
Thinning out your userbase too much is definitely something that many other games have had to deal with. People can be picky about the cars they drive, be gate keepers or judgmental about the way people should engage with the game, only have specific times they are available, etc, and that naturally splinters groups and interests, and it's up to iRacing to decide who they're going to force together and in what ways.