Remember that, while it isn’t strictly necessary, corruption and cronyism occur very commonly in fascist regimes. Fascists don’t just punish their enemies, they also love to reward their allies. And they need allies. You can’t make it to becoming supreme leader of a country all by yourself. No fascist is an island.
That means there are people out there for Mueller to flip. And when Mueller flips the President’s personal lawyer, that’s big news. Especially since Cohen pleaded guilty to committing crimes at the direction of Donald Trump, specifically to influence the election.