Forza Motorsport unveiling was actually two years ago as part of the Xbox Games Showcase, not last year or technically not E3.
This was back in that awkward pokerface period where PlayStation was still claiming it
wasn't cross-gen because it believed in generations, and Xbox was trying to recover its messaging after proudly announcing it
was cross-gen because its scaling approach was so solid that every release could be the best the platform could offer. (This was back when it had up and later removed a lot of "Coming Soon for Xbox Series X & Xbox One" messaging from like Everwild and Avowed,
not specifying if those labels were correct for the projects at the time or a mistake; the company was also still foggy about
how Flight Simulator was going to be released.)
It feels like since then, the cross-gen battleground has been less bloody, as cross-gen scaling has proven itself (the two Horizons, FH 5 and Horizon FW, are two of the best-looking and performing games on any platform) and the next-gen phalanx is holding for UE5 and some other technologies like Primitive Shaders/Mesh Shaders to be worked out for practical purposes. Of course, everybody still craves that "real next-gen" experience to blow them away, but the reality of why that didn't for the most part happen in 2020 or 2021 (depending on how you look at like Ratchet or Flight Sim) and still is mostly a promise of things to come in 2022, hopefully people are getting that?
...So, aside from the LOLs of the situation flipping again and the backlash-against-the-backlash of GT7 going cross-platform after its initial "Only on PlayStation 5" designation, I think Forza will be fine either way.
They'll be able to do great stuff with it on Xbox Series, and there's still room to take most games down to Xbox One (particularly if they port it and maybe outsource it, like FH3, rather than having the main team trying to make two games in one with a single engine.) I'd love for the Forza 8 engine to be capable of things only possible on Xbox Series X/S and high-spec PCs... but then I look around at what's going on in game development and don't really see what that would be right now.