Fortunately, there is no statute of limitations on tax fraud. The IRS has infinite time to assess extra taxes and apply a 75% civil fraud penalty. We just need to elect someone with the will to do so.
It will require a new administration to hire a cadre of auditors to go through all those returns. I read that increasing the number of auditors paid something like 10x the salaries of those auditors. That number might be off, but it was many times the salaries of said auditors.
State departments—such as the California Franchise Tax Board (FTB)—often have a much longer collection window, such as 20 years from the date of assessment.