I would agree that Persimmon have no obligation to replace the sealed units or pay for the replacements as it is now outside the 2 year warranty.
The NHBC are also correct, as this is not a structural defect as prescribed in their Buildmark warranty. However, most sealed unit double glazed units come with a TEN-YEAR guarantee.
You should therefore take this matter up with the window manufacturer directly. They in turn may claim against their supplier of the units if they did not make them in-house.
If the glazed units do not have a ten year guarantee, then all you can do is highlight the problem with Persimmon CEO Mike Farley and hope he considers the bad publicity you could generate for his company would be more costly than doing the right thing and changing your sealed units.
As this is a development wide problem, it may be the case that some houses may have reported a problem within the two-year period and this could help you validate a claim against Persimmon under the 2-year warranty.
I would agree that such systemic failure of the units should not be expected so soon and in such numbers and this would indicate a failure of quality control with the original manufacturers of the sealed units. However, it is equally likely that they made not have been fitted correctly by the installer.
Reputable firms are FENSA approved and members of the Glass and Glazing Federation.