I actually watched a video about this that kind of explains the divide and why it happened. Sonic Adventure is mostly immune to this since, while it does deviate a bit, it still mostly adheres to the Classic aesthetics; there are still badniks, the aesthetic is still pretty surreal, and the story more or less serves as a direct sequel to Sonic 3. Its easy for a Classic fan to rationalize Sonic Adventure and Sonic 3 are within the same series. Then you get to Sonic Adventure 2; suddenly everything looks super photo-realistic, all of the Badniks are replaced by generic government robots in almost every level, and the story has very little in common with the Classics, instead focusing on Sonic's new, darker rival and suddenly there's a government that exists, etc etc. All of that by itself would be fine, but Sonic Adventure 2 is kind of the point when the "Modern" aesthetic took over while the Classic Aesthetic was being phased out. Its like the transition of Dragon Ball to Dragon Ball Z, and when Goku grew to being adult. The OG Dragon Ball was gone after that, while the Z version became the "default". So to many Classic fans, it felt like Classic Sonic was being "replaced" and the Modern version was piggybacking off the success of what Classic had established. IT didn't really help the games that would follow the Modern path kept getting more and more divisive, which validated those feelings. That's why so many were happy when Classic returned in Generations and Mania soon after; it felt like a return of "their" Sonic. If this was directed at me, I'm purely speaking for myself here. And I'm taking a firmly neutral stance at the time being. Like, if y'all wanna get excited or be negative, be my guest, Imma just be in the corner lol.