Didn't Sega/Paramount say they do not intend to recast Dr. Robotnik?
And why would they need another actor to play as old man Robotnik when Jim Carrey already played as an old man Robotnik?

Because it could have been different universe too, not just future timeline.
If it's from different universe, it could've been played by another actor.

It could also used to explore the dark or tyrannical version of Robotnik.