Oh, I don't know.
The Old Rite was available here in Milwaukee before SSPX, too--but only because there was a "wink-wink" arrangement with R Weakland (of ALL people!!!)
But I think the case could be made that JPII wrote the Indult because he was concerned that SSPX & Co. were becoming "an alternative" for a number of otherwise-faithful RC's who were NOT questioning other provisions of VatII, nor were collecting memorabilia from a certain German regime from c. 1932-1945.
No, SSPX was not the Single Influence. But it had critical mass--moreso than the here-and-there onesy-twoseys around the US and Europe.
The Milwaukee Archdiocese in the days of Weakland and of Cousins always had great Catholics like ninenot and may have had Tridentine Masses but I don't think that you would argue that, whatever the sins of the AmChurch, Milwaukee under Weakland was not in particularly scandalous shape.
Did Weakland actually give permission or did he ignore the Tridentine Masses?
As to SSPX reaching critical mass (they were certainly critical in the whiny sense and they certainly said Masses but the combination does not mean that they reached critical mass)
You are also forgetting the SSPXers who probably think that 9/11 was an "inside" job or that Williamson is a collector of such memorabilia or at least a denier of historical truth as to the misbehavior of same.
God bless you and yours!