They've already made Sunset Heights and Chaos Island for Shadow Generations. They could've just copy-paste Sunset Heights and import Chaos Island to Sonic Generations and modify it for Sonic.
they would just need to work on a stage representing Sonic Lost World then.
While that's the ideal, like I said before they can't add anything in Sonic Generations in the given time.
The people who worked for remastering Sonic Generations and making a whole new game called Shadow Generations were different.
Besides, Shadow Generations was the only incentive to buy Sonic Generations along with it. If they were willing to rewrite the script for the game, then they could've added more to it if they wanted to.
"Shadow Generations was the only incentive to buy Sonic Generations along with it?"
Not really.
People have been requesting remastered of Sonic Generations after Sonic Colours Ultimate, especially on Nintendo Switch.
The demand was really high for that.
They're still gonna buy it whether it has Shadow Generations or not, me included.