There's no need for a 150-200 development team for a Sonic game, sure, but Sonic Team should actually take the time to finish their work rather than rush parts of their games to get them out in time for holiday. Their games being unfinished is one of the biggest reasons for why the brand is as damaged as it is.
While Colours and Generations fixed it up somewhat, they weren't good enough to stop people from people of being doubtful if the next Sonic game will be good or not. After that the mixed reception for Lost World and the horrendous failure that is Rise of Lyric only brought the brand back down again. Sonic Team's next game needs to be great, or Sonic will continue to be seen as "that crappy 20+ year old mascot that's just a mangled corpse held up by strings".
SEGA can't afford to rush their next Sonic game out for holiday release, they should release it in 2016 if they have to. If it actually looks and plays great, they could even hype it up as the big 25th anniversary game and get extra sales by doing that. Either way, they need to put out a game that's great. Not one that's filled with filler put in there to make the game longer, not one that's so short that it's hard to recommend for full price, but a full game that's at least mostly fun to play. Sonic Team can make a game like that, they proved that with Generations. But they need enough time to make a game like Generations that doesn't take two hours to finish.
It seems like the Generations team has had three years up to now to make next year's Sonic game if they started after they finished Generations, meaning that they'd have worked on it for four years when the game comes out. That should hopefully be enough, but it seems likely that they'll have made a new engine to take advantage of PS4/Xbox One hardware and that the gameplay will once again be different. If that's the case, they need to take all the time they need to polish that up.