I don't think the breakdown of modesty has contributed to the breakdown of the family. Instead, I think our dress is a reflection of our attitudes. It is a symptom, not the main disease. When our attitude toward God is wrong, it distorts our attitudes about marriage, modesty, etc. It is the heart that chooses to do right or wrong.
You asked if pants on women were ever considered immodest. Ironically, you also mentioned that they were used by women for modesty at work during WWII. I think that Hollywood women first started wearing slacks and society was shocked and scandalized by their boldness and rebellion against established tradition. These gals were considered brazen, bossy, and feminists. They had little regard for morals, and people identified them by their slacks and cigarettes and daring bathing suits. Their clothes did not make them rebellious, but rather, their attitudes controlled their modesty standards.
Now take a Christian young woman on a farm who wears slacks to milk the cows and muck out the barn. It's easier and more convenient to wear slacks so she starts wearing them to the store and around town. Then some pious skirt-wearing believers start snubbing her for being "rebellious" like the Hollywood crowd. They judge her clothing, not her attitude. They start putting more emphasis on their good work (wearing skirts) instead of the heart and consider themselves better Christians. And so it continues until the younger generation just can't understand what all the fuss is about.