Flynn’s credibility as a witness is completely destroyed now that he’s been convicted specifically for lying. To be trusted again he would need verifiable audio or video evidence that may not even be admissible.
The goal here isn’t a criminal conviction. It is to damage Trump politically. Paralyze his agenda and get him to not run again in 2020.
Their original goal is to get Trump on collusion (conspiracy) with the Russians. If that fails then obstruction of justice for telling Comey to go easy on Flynn who was eventually charged. This is far from over, just hitting speed actually.