I understand what you're saying, but in your original post you wrote:
FAR 4.804 requires FFP contracts to closeout within 6 months. Cost type contracts should closeout within 3 years.
If the contractor's failure to meet these FAR mandated time standards are because of their failure to submit a final voucher/invoice, then the Government should unilaterally closeout those contracts.
The time standards at FAR 4.804 do not apply to contractors.