The thing with league racing is that it's more about the group of people than it is about the game. You can have great league racing in pretty much any game, because the whole point is that you have humans setting the rules and doing all the hard work to make sure the races are fair and fun so there's very little for the game to do other than be drivable.Sport mode is decent I have had great races, and bad but where GTS shines in my opinion is these online leagues on GTP! From getting your car livery ready for opening day, caution laps, clean accountable racing, Full damage, fuel strategies, pit strategies, rivalries, and much more. The experience you get racing with a group of good people who compete fairly in my opinion beats any type of offline play.
As far as online being better than offline... if you're a competitive racer, then sure. But for people who it's their jam, as many people above have pointed out, then it's just not fun for them.