You could easily do a story of Robotnik dangling a false flag of him being alive and simply imprisoned, playing on their guilt to bust him out and it being a trap. Maybe instead of him just being a robot he would also be aware and talking but unable to control his body to twist the knife.
I like the first idea. Sounds like the cruel thing Robotnik (or rather Snively) would do.
I'm not so sure about the later though. It seems kinda redundant when Uncle Chuck already has that role.
The interesting thing about the concept of Robotization is the sheer amount of possibilities you can explore with it. For instance, what if there's a Mobian character who prefers being a robot? Perhaps they were deathly sick in life, or possibly disabled; no longer feeling any pain and discomfort sounds like a good reason to remain roboticized. Or maybe they like the feeling of strength and invulnerability, and they go mad with power because of it.
Another interesting idea is the logical implications. Like, what would happen if a Robian's mind was placed in another body? If they were restored to normal, would they be mind-swapped? This could be a cleaver way to incorporate transgender characters. Or heck, what would happen if you built a Frankenstien-inspired chimera out of spare Robian parts? The ramifications are quite frightening!