The feds could protect him physically, no question. The NSA and FBI could destroy any mob that threatened him, for example if he was a crusading DA going after the mob. But that's not what he is, he was their lawyer for decades. He knows where bodies were buried, and they know things on him that he never wants to see the light of day. He could sic the feds on his associates, and they might go to jail, but then the mob would release a storm of information on him that would be devatating. He can't have that, and so he has to accept their beat-down without fighting back.
That's my theory, anyway.