I believe that Steam would certainly sell a lot of licenses and bring many new players into the game, though that may not translate necessarily into improved attendance at events in the long term.
While Assetto Corsa sure does have the bling and modelling detail to attract a large audience, its physics still leave a lot to be desired, at least in my opinion. To me AC still feels like its predecessor; Netcar Pro, with it's general 'understeery' attitude towards rear wheel drive cars. I just can't seem to escape the 'pole up the arse' handling even though AC is certainly an improvement on its ancestor. But the point remains that better graphics and more polys are the expectation of the new customer in the modern era. The upcoming facelift for LFS is sure to appeal to a greater audience.
For many of us I know that good car and world physics trumps high-poly, fancy graphics every day of the week. It's about the actual racing after all, and Live For Speed has never been found wanting in this aspect. Even Live For Speed's tyre model seems today more sophisticated than its modern rivals, and (for me at least) the force feedback - while not perfect in every way - is the most intuitive to be found when it comes down to knowing the limits of your car. The netcode is solid and the rapidity with which one can fire up the game and be racing online remains unrivaled.
Live For Speed is an excellent racing sim, and although I'm sure I can't completely shed my bias, I'm certain that my mind isn't playing tricks on me, and won't be easily changed, either.
Having said all that; I've always believed that any game endures not by virtue of its features, but by the passion and efforts of its community. Pity that more developers don't (won't) grasp this important concept. While LFS will garner more appeal from graphics and physics improvements (and undoubtedly by the addition of more content), at the end of the day it's the players that hold the future of the sim in the balance. A loyal and dedicated community of racers is the answer to longevity.