Personally I feel that Horizon captured the whole driving around in tuned-up road cars side of Forza, giving it an open world and slightly more accessible physics was a great match. For me, having lost that side of the series Motorsport needed to be a bit more gutsy about the focus on motorsport, and I was disappointed when FM7 tried to mash together upgrades and motorsport into some horror show where a 1930s Bugatti had wide slicks and big power forced upon it to compete with cars it shouldn't be competing with.
I only started FM8 just before the addition of the Champion's Cup, but the Builder's Cup as it stands now is at least a better attempt at motorsports-ifying the upgrade system, with a limited budget that grows through the series. It lets itself down though by applying that upgrade system to the rest of the game too, and by performance-balancing the AI so upgrades aren't even needed to compete. The Champion's Cup is much more like I pictured the post-Horizon future of Motorsport to be, using the strength of the car list to give a long list of racecar-specific events with no arcadey upgrades. If they'd launched with that to begin with and focused on dethroning Grid 2019 and Grid Legends, for me at least it would have been a stronger product. It certainly would still be controversial to drop the road cars, but it's always felt inevitable to stop it feeling like a Horizon variant.