"Existing common shareholders do not own the companies at all. They have no economic rights and no voting rights. Nothing."
If you want to make an extreme click-bait statement, sure.
But as usual, you state facts yet drill down to a single conclusion that only time will tell if it's true. We are still shareholders. Shareholders have the rights associated with the Shareholder agreement. These rights do not dissolve suddenly in 2012 because of a contract between two government entities that violates the fair dealing with Shareholders. These rights travel with the shares. This is backed by the Berkley verdict.
The economic and voting rights are temporarily suspended during conservatorship. That is why it is a Conservatorship and not Receivership. Dilution of those rights is possible as part of the emergence from Conservatorship. In fact, Treasury could wipe all of the Common and JPS value based on their LP amounts. It's also possible they don't wipe 100% of the value of the companies. And no - the amount of equity Treasury took from AIG has zero bearing on what they may take from the GSEs.
Furthermore, FHFA (control) and Treasury (ownership) fall under the Executive Branch. As such, the control and ownership can be altered based on political winds of who is in charge of the Executive branch at any time. It is possible the leader of the free world will abscond with the value of 2 publicly traded private entities worth billions of dollars. It's also possible the leader of the free world will believe in the 5th Amendment. We may see a concept similar to Tyler v. Hennepin County, where the GSEs would be seen as obligated to render unto Caesar what is Caesar's, but no more. The resolution of Conservatorship may include NOT further violating the covenant of good faith and fair dealing with Shareholders. IMO, this would be the smarter resolution.
It is completely unnecessary for Treasury to liquidate the GSEs in order for them to function again or to be rehabilitated. There is no need for a government windfall to make Caesar whole. So while I agree with you that it is possible Treasury conversion wipes common shareholders out completely, I also see other possibilities. If you don't see those possibilities, that's on you and I'm sure you will invest accordingly.
