I don't know how accurate that graph is, but taking it as accurate, I think it's a normal decrease and stabilizing process of any game. Close to launch, lots of people play it because it's new, then move on to other games.
If >100.000 people are playing GTS Sport Mode on a weekly basis, that's not a bad number considering lots of people simply avoid Sport Mode and only spend time in lobbies (leagues).
Also, worked for what? Attracting more people? We don't know, because active users would decrease (as had been decreasing since a few months prior to the change). If it worked for me? Yes. The week I played more sport races was a week PD implemented open setups and Race C was around Nurb GP with Gr.3 cars. Then daily races became daily and I stopped playing sport mode.