AFAIK, Plaintiff's did not use estoppel claims in those cases. The story is two-fold, as wom_ri noted (or three-fold?).
8 usc 1255 could be interpreted this way: a visa number need to be available when I-485 is filed and should be allocated when I-485 is accepted by the USCIS. When I-485 is approved, visa number is subtracted from the pool of numbers for the fiscal year when I-485 was originally filed ("which the alien is chargeable for the fiscal year then current" 8 usc 1255(b)).
This argument can be useful even for filing WOM when your PD is not current.
According to 8 USC 1153 (g)- if a person fails to apply for immigrant visa within a year after s/he is notified that a visa number is available, s/he is no longer eligible for a visa. However, if a person shows that the failure happened because of the circumstances beyond alien control, a visa number can be reinstated within 2 year periord.
I filed I485 on September 2005 and a visa number was available then. But according this "2 year period" rule, I can not claim that visa number after September 2007, is that right?
We can also argue that current regulations regarding priority date, based on LC filing date, not on I-140 filing date are invalid.
This one doesn't apply to me since I am in NIW and later EB1A (pending).
Last, but not least, we can argue that nothing explicitly prohibits issuing visa number even if PD is not current according to visa bulletin (many examples when USCIS did that and indeed, the INA says nothing about visa bulletin).
Visa bulletin is being used to estimate visa availability.
I heard that some IO don't follow the visa bulletin and issue green card even the PD is not current. Do you have some specific examples that we could use as evidence ( I think there is some rule about "hearsay", just learned that from LAW and ORDER)? like case number or a person with a specific name?
If court does not accept this interpretation, we can argue that the government should be prohibited (estopped) from denying visa availability as we lost eligibility for visa number because the government violated federal laws.
In the favorable case 1 page before, it state" The visa # ran out as of July 2nd, 2007.Heinauer Decl. P8. The responsibility for the lack of visa #, is not attributable to plaintiff. In this case, defendants are entirely responsible for the delay in processing plaintiff's application."
Does this mean that the court grant estoppel although the plaintiff didn't claim for it?
Finally, AC21 recaptured unused visa numbers and about 100,000 where unused (and I believe, are still available) after FY 2003. See
http://travel.state.gov/pdf/FY2003 AppD.pdf
So you can argue that visa numbers are, in fact, available.