Cool topic.
Briefly, to get to where LFS would be watched by an audience, it would have to present something unique and/or significantly interesting compared to what is already out there.
As someone who spends a lot of time observing LFS races, I can tell you that we aren't there yet.
I love watching live races, and I think it's simply amazing to be able to hop from one car to another, with unlimited camera angles (not yet perfected but you get the picture). Also, the LFS community has some incredible talent on the side of the drivers, and also on the side of developers who create the environment.
Having said all that, I only really appreciate the racing because I know how hard it is.
In real life, I bet half the crowd only goes to a race in case there is an accident, so they can see something thrilling. Just having a thousand kilo hunk of metal flying by you at an astonishing speed is a thrill. LFS is on a computer screen. (also sitting trackside and watching cars go by in a live race is something of a stutter-fest at the moment, at for me on my sub-par machine)
So where does that leave us? We already have a great base: the environment, the driving talent. What we need is something unique, something people will be thrilled by, even if it's on a computer screen, and the human support side of the broadcasting.. the organisation and commentary, and the sponsorship hunting etc.
Personally I think if it will ever happen, the key will be that we as sim racers are not limited by reality. We already see this a bit in the way we can have unlimited camera angles to observe the race. From LFS's spirit so far, we can gather Scawen doesn't intend to publish billion horsepower cars, magnetic wheel cars, thousand foot jumps, space driving, etc etc etc as part of the LFS universe. So to attract people.. hmmm
mortal avatars?
something at stake during the race?
exceptionally dangerous courses that drivers would be too afraid to try for real?
some link to reality for comparison with real racers?
Ok ramble mode off.