Yeah, I'm thinking that might be their plan. Have this one single Forza Motorsport game, updated with DLC for the rest of the generation, much like Halo Infinite is planned to be the only Halo this generation with DLC support for the whole gen.
The Windows 10 strategy. I mean, they are already doing it for the consoles. Makes sense to release "Forza Motorsport" and just add stuff later.