discriminate against GSE shareholders based on the date they purchased their shares
Interesting argument. In most such cases this is the appropriate action - the damages are awarded to the shareholders of record on the date their shares were impaired.
However - this is an unusual case. The impairment happened in several steps, making this much more difficult.
And - I would bet that the majority of those who bought shares after the NWS started believed the NWS would be either temporary, or found unlawful. So the impairment happened over time as the NWS persisted and the suits were unsuccessful.