If Gambino had lived another ten years,assuming he remained mentally and physically healthy,he would have ended up dying in bed just as he did.He wasn't going to get caught on wiretaps,he didn't associate with most of the members of the Family,and I doubt that any potential witness against him would ever live to testify.
Carlo was the prototype old school,low-key Boss.He wasn't flashy,didn't even own a phone,and maintained a very strict chain of command.
RICO only works if you have tapes or witnesses,and in this case,the Feds would have had neither.