Lefebvre relied on traditional attitudes which over time have been softened and eroded by many of his core supporters and benefactors. New generations like the 'benefits' of modern life and would amend principle for them to continue.
I don't think this is true.
I remember attending SSPX masses in the UK 1978-2004, US 1997 to 2008, Australia, 1994 to 1996 and France 1984 to 1998 and attitudes were not more Traditional than they are today.
For example, I never heard Archbishop Lefebvre or any SSPX priest in the late 1970s or early 1980s suggest that women should not go to University. My sisters all went to University around that time and none of them ever had negativity from any cleric or lay Catholic. That is a value or attitude that came along in the 1990s and I believe came straight from the pen of Bishop Williamson.
Modesty in dress was mentioned from around 1982 but notices pinned to church doors and laity being called out or refused the sacraments did not happen until the late 1980s at the very earliest. Lefebvre was dead or just about to be by then. Those attitudes did not come from him, but directly from the United States priests of the SSPX. I know because I first visited the US in 1997 and went to mass in 14 different US states and the dress code was an abiding memory.
I remember many young women and girls wearing trousers to mass in London or Paris between 1978 and 1986 and unless they were skin tight they were rarely if ever criticised for it.
Your mileage may vary, but Lefebvre was a Frenchman living in Switzerland and my Eurocentric memories of meeting him and visiting the chapels and priests he was closest too was that they were not as hard as the hardliners today. The aggregated Pre-split SSPX was far more hardline than the aggregated SSPX in Lefebvre's time.
I think Lefebvre actually relied on hope, (it had only been a decade or two that the crisis had been going on), Charity, he understood it was better to have people in the SSPX chapels with liberal faults than to chase them away, and much wisdom and common sense. I remember a lot of talk about the Freemasons but nobody ever doubted the moon-landings or helio-centrism back then. I was a very keen science student in those days so I think I would remember. Back then nutters were not mainstream.
I think several things happened to change the nature of the SSPX
Assisi, shocked and scared people and made them close ranks and react in an opposite fashion.
The hardliners got more of a foothold when Lefebvre was dead or dying and a group of less hardline SSPXers left the chapels. Various priests left to join the FSSP to escape the hardliners. Around 1993 the SSPX in the US was about as hardline as it has ever been. Converts arrived who adopted the more hardline status quo, as newbies they had no other option and converts find the zealousness attractive. Bishop Williamson who had always had a loyal camp of seminarians and priests, who liked his charm and oratory, picked up a group of lay supporters too. As the only English speaking Bishop, Williamson had a distinct advantage over a secretive Swiss and a couple of other Bishops with almost no public profile.
The h0Ɩ0h0αx comments and other things Bishop Williamson had said before brought matters to a head and Bishop Fellay could see he had to do something to stop an internal war.
The reality as I had experienced it, is that the arguments the SSPX used to justify its position 40 years ago now appear weaker and the situation more confused than in the early 1970s.
The next big event will be the Canonisation of JP2 at the end of April. If that goes ahead then I know very many Trads from FSSP to SSPX to Resistance who will be very troubled by it as they were by Assisi.