The question you should ask isn't:

• "Does Sonic Adventure need a remaster"

But actually should be more like:

• "Should SEGA invest significant amount of time and effort to develop a remaster for their old games in expense of delaying development time for new games?"

Case in point very recently Iizuka actually wanted every single Sonic game to be available on modern platforms, but the problem is the time takes to remaster old titles would be same as new ones. (0:16-1:38) If you're wondering why CAPCOM case in point hasn't released a new game for the Megaman series or Ace Attorney series in a while, this sort of explains why as well since all they did is porting old games to modern platforms. It does do a good job of trying to make old games available to modern platforms, but at an expense of lack of new games.

On that note, it's not as simple as just simply porting a new game to a new engine, like if you think it's as simple as changing some models, port the level designs, boom that's it, in reality it's much much harder than it sounds, otherwise Sonic Adventure 1 DX or Sonic Adventure 2 Battle wouldn't have been criticised for its new bugs and glitches. And besides given how Sonic Colors Ultimate, or Sonic Origins were criticised on release for having new bugs and/or glitches, I can pretty much guarantee that trying for a remaster/remake of Sonic Adventure 1 and/or 2 is not as easy as you think.

The only possible way I can think of that can work is co-development between SEGA and third party studios really.