Well, not specifically to wipe out shareholders...
The way I see it,...
When Franklin Roosevelt first created Fannie Mae, it was a government entity. That eventually went away, but the original plan was that the government would have direct influence over its operation.
THEORETICALLY, if they wanted to return to a model with direct government oversight, it would be nearly impossible to impose such a directive over Fannie Mae. So instead, they could attempt to create a new entity while the GSEs are stuck in Conservatorship and then force a sub-cost mortgage sale from the GSEs to the new Government Entity under the rule that the Conservator has "extraordinary powers". This would leave the existing GSEs worthless without any mortgages in its portfolio. The GSEs would then get released from Conservatorship and shareholder rights would get restored over the worthless companies. The new entity would then take over the market under government control.
I want to make it clear. I'm not making a prediction here. This is only one theoretical outcome among hundreds.