Think about the thought process, and the feelings, experienced by a person who "softens" his stance on Vatican II.
For example, let's think about a Traditional Catholic who joins a Novus Ordo Catholic book publisher. He gets hired, and finds himself among lots of Catholic co-workers. He has friends and camaraderie. For once he's not considered "nuts". He feels "moderate" and reasonable. (This can't be understated -- for one who has Fought the Fight for decades, it can be like a breath of fresh air, or a glass of ice cold lemonade for a man dying in the desert.)
One day he notices that they're selling a book on "John Paul II the Great". Is he going to bring up the problems with JP2's pontificate? Probably not. He's more likely to find ways to excuse everything he did, overlooking the bad, and trying to cast him in a bright light.
This applies to countless people in the SSPX, as they are faced with the reality that the SSPX is softening its position. Human Nature says "All RIGHT!"
It's a scientific fact that most people are going to go with human nature. Heroic virtue is always heroic and rare. Most people will go with the flow, the path of least resistance. Even most Catholics trying to stay Catholic will go out of their way to reconcile the world with the Faith. They will try to have it both ways.
So most people aren't inclined to Resist anything; they would rather go with the flow. No wonder the "Resistance" is going to be more or less capped in its potential membership.
Remember: Emotionally and psychologically, all men want to be loved. Not just by God, but by their fellow man. I'm not saying that some of us aren't trying to be heroically faithful; I'm saying that it COSTS US when we do this. It's certainly not going along with human nature. Men DO want to be loved and appreciated. To give this up is to make a sacrifice to God.
I've said it a million times: heroic virtue is rare, that's why it's heroic. Saints/heroes are looked up to because they are special. If everyone could be a hero, heroes would be nothing special.