Forza games individually don't sell much, H3 being the only notable hit with 2.5 million. Compare that to GT6, late PS3 games that has sold near 6 million. The points you made can be made to the PS360, where GT5 released late, and managed to sell 10 million+. These games are incredibly popular in Europe (as is racing), though I don't expect GTS to do as well as GT5 since its more online focused.
Being more online focused usually means the game will probably last longer due to its online services for competitive gaming communities, which means it could get exposure on Twitch for longer and have a longer shelf life. Especially if they don't end up releasing a new Gran Turismo for another four years.