If the content stopped, then it stops. For Sport Mode, it could have stopped one we got all the Gr.3 and Gr.4 cars we needed. The core of the game are FIA Nations and Manufacturers Cups. For us mainly offline players, it's a thumbs down, but that's what GTS is about. A next phase for the franchise. Maybe be a hard pill to swallow for many players.
Again, there are plenty cars in those two Groups to use in this game. Similar to real world racing in FIA and SRO sanctioned series. Look at real world GT3 & GT4 racing(which GTS are mimicking) those are the only cars that can be used year on year.
It would be interesting if PD swapped out certain Gr.3/4 cars, for updated ones, that were new or evolutions. It would only apply to new seasons not your personal garage(in case players bought cars with real money and /or Cr).