Hasn't it always been the Robotnik bit that he genuinely seems to explode but he has always inexplicably escaped death? His machines always explode with him inside, and we never questioned him showing up in the next game. In Lost World he fell through the atmosphere and hit the planet, only to get dug up by his bois so they could keep scheming.

I think Eggman's true superpower is always having an escape plan and an absurd amount of durability. They made it very obvious that he still had the Sonic Quill, which also turned super sonic when he did. And tbh it would be classic Eggman for his version of being nice in a breakup would be giving Stone the validation he wanted and faking his own death so he could move on to scheme on his own. He's smart, it wouldn't be above him to learn the wrong lesson but still understand the lesson other people would expect him to learn. He just got betrayed by his own grandpop. I would have expected him to burn all bridges at that point anyway.

At any rate, I know this character doesn't exist without Carrey but I also think Carrey has a blast playing this character. I think they are using the fact that Robotnik always apparently dies (or at least definitely should have) but doesn't, to keep from writing themselves into any corners should they need to do without him.