He would probably become worse. The only thing that would be useful would be the computer for analysis (not chess.com computer).
imagine if fischer or any legend of old had the chess.com app on their phone..

...or even if they had a cell phone.
Fischer would have been too busy tweeting to study chess.
Interesting question.
I suppose a guy like Fischer would have been even stronger with support of a modern chess engine. And think about the genius of Capablanca: he was so naturally talented for chess that he could have mixed his intuition with exact analysis...
But I wonder if other chess personalities like Michail Tal would have lost some of their attacking istint. Some romantic attitudes of the past now show limits and mistakes with modern computer analysis: could it be a limit for their ambassadors in today chess scene?
how great could they have been?? what awesome times we live in at the moment