Depending on what and how the FBI screwed up, it's actually possible today. It's innocent until proven guilty, and if they screwed up in obtaining a key piece of evidence, then it and anything stemming from it will be inadmissible in court. If they can't prove guilt beyond a reasonable doubt without that evidence, then the defendant will be found not guilty.

Anything is possible, but that doesn't mean it should be automatic as proposed.

Yes it does. Especially when the nature of the evidence is exculpatory, and was in the State's custody at time of the proceeding, and maliciously destroyed by a State actor. Adverse inference is a two way street. If you want to use it as justification to help the Prosecution when a Defendant destroys evidence, it must work the other way around as well. Due Process was thwarted.

>evidence is exculpatory

So now you potentially go to trial with that evidence.

How exactly is that possible if said evidence has been destroyed by a state actor?

Shades of Alex Jones' lawyers asking for a mistrial due to their own screw up.

Jones's case was civil, where the burden of proof and the penalties are remarkably different.

Had Jones's case been a criminal trial, the lawyer's screwup would have given Jones a powerful Fifth Amendment protection argument for excluding the relevant cell phone data from the trial. And anything that is found in that record is unlikely to be applicable to any criminal proceedings against Jones in the future (with the exception of perjury; I'm actually unclear on how a charge of perjury works in this circumstance when the Court is holding evidence provided by the defendant that the defendant perjured themselves).

Very much in the spirit of

"This isn't fair to me."

"I propose a solution that isn't fair to everyone else ..."