I'm not so certain on Motorsport reboot in 2021 personally. According to an interview, it only began full development in December 2019, before that they were mainly working on the new ForzaTech engine and working on DLC for Motorsport 7 and Horizon 4. A November 2021 release would give it less than 2 years of full development, and since they're starting over from scratch with all new tracks and all new car models instead of reusing tracks and cars from Motorsport 5-7, they'll need extra dev time to get enough cars and tracks ready to satisfy people, they don't want to run into a Motorsport 5 situation again with critics attacking the game for not having enough cars and tracks on release.
In theory at least, Horizon 5 should have been in development longer, assuming they put it into full production as soon as Horizon 4 released, September 2018. So Playground could get a cross-gen Horizon 5 running on the old build of ForzaTech out in 2021, but if they decided to wait on Turn 10 to finish the new build of ForzaTech, and engine that wasn't ready until late 2019, we might not see any Forza game in 2021 sadly.
This is the earliest we've ever seen a Forza. I know they said it's early in development, but considering they could pump out a new Forza every 2 year prior, I'm certain they can get another one out by next holiday.
I'm no game engineer, but I would find it strange if they scrapped everything they had prior and at least didn't use some as car/tracks assets for the new engine.