Where is the logic behind this? I'm legitimately curious.
There's been a good number of references to the Adventure series this year. Most notably, the "Sonic Adventure Music Experience" that was held in Japan. Also, that PSO2 event is supposed to have Sonic Adventure music and references too.
The thing about the 25th Anniversary though is that it's obviously gonna give some spotlight to all branches of the franchise (Classic/Adventure/Modern/Boom), so it's really hard to speculate what the game will be like.
Personally, I'm betting on Classic Sonic being involved somehow. I think they are gonna bring him back, if not as a major part of the game's premise, then at least as an unlockable or something. He's been all over the merchandise and even the memes as of late.
There was a huge demand for him when he popped up on the roulette in Sonic Runners, so I think SEGA are starting to see they should use the design more often.