The agency has withheld and destroyed evidence on numerous prior occasions, and it has spied on the Senate's own investigations. A couple of generations ago Harry Truman, on whose watch this damnable entity was first chartered, recommended that its powers be curbed. Interestingly enough, this was exactly a month after JFK, who had earlier threatened to "splinter the CIA into a thousand pieces," had his skull splintered into several dozen. And in the aftermath of that little exercise of the Fourth Branch of Government, AKA the Lone Crazed Gunman, Central Intelligence withheld, covered up, obfuscated, misled and lied outright six ways from Sunday, impeding the investigation and doing everything it could to divert attention from from the improbable number of Langley assets that were connected in ways large and small with the assassination.
The IG is outside the normal chain of command. It's not in his interest to try to destroy evidence, especially evidence that exists elsewhere.
Your faith in the disinterestedness of the IG is touching. Nice little backups the agency maintains "elsewhere." Be a shame if something should...happen to them.

cordially,