Both Fannie and Freddie have been growing and now effectively back ~$5T of MBS.
R-ship is not an option (its also why USTSY took 79.9% warrants and left stock outstanding). No R-ship until a proven third entity CSS LLC (CSP) is fully functional and proven. That will take several years to a decade and at that future time F&F would need to stop writing/securitizing new business.
At that transition point, it would still take several years for their remaining portfolios to wind down. 10-20 years in total duration
If the warrants get rescinded by court action, all your bets are off.
Lots of variables to consider in weighing possible outcomes. One such alternative would be release and juniors paying a 5% divvy going forward as the S/P returns to $20.