He'll get his day in court
The FBI doesn't need to show all of their evidence to have Dotcom extradited. He'll get to see all of the evidence long before he gets to trial. All the FBI or other proper authority needs to do is present reasonable evidence of the perps alleged crime. Extradition is not a conviction, it's part of due process. Once he's extradited due process can continue and the FBI must prove their case. Considering how easy Dotcom has made it for authorities, that won't be difficult. The only qustion is will the punishment be appropriate or just a slp on the wrist?
Japan has made the process crystal clear on digital crimes. You pirate you spend 2 years in prison plus a fine. You facilitate piracy as Dotcom is alleged to have done, you spend 10 years in prison plus a huge fine. All countries should adopt these mandatory punishments so there is no place to run to or hide after committing digital crimes.