Everyone has their own self-diagnosis for Sonic's downfall - when it happened, why it happened, what exactly decreased in quality, so on - but I think this gave me a lot better an understanding of Sonic's modern problems: Sonic makes big money in the West, but the Japanese audience just sees him as Sega's corporate mascot (like Mickey Mouse for Disney over here) - so regardless of dev interests, Sega's higher-ups don't want "good Sonic games" - they just want "Sonic games". While that is just a theory it certainly wouldn't be surprising with SoJ and SoA's history of in-fighting.