You can be contemporary yet modest.  I think those who dress "old fashioned" are just trying to call attention to themselves.  "Look at me, I hope you will think I am holy because I eschew contemporary clothing".  

Why is the wearing of modest contemporary clothing so important to you? Why must the modest clothing be contemporary?

People have forgotten how modest Catholics used to dress to mass, so they copy the dress styles of other very conservative religious communities. 

Back in 1994 there was a huge group of SSPX homeschoolers who decided to move en masse to Idaho because they wanted to be near a school staffed by SSPX nuns. As they explained, their older high school children no longer wanted to homeschool but wanted to attend a regular high school. However, there was no regular high school they could attend except private Novus Ordo schools, private boarding schools, or public schools, all of which were teaching abortion.

This particular group dressed very modestly and reminded me of The Little House on the Prairie, but my young son was really turned off by their attire as the older girls could not really play sports in fear of tearing their dresses. Likewise, the boys feared getting holes in the knees of their slacks, so these children were prevented from playing outside.  Thus, in the words of my son, the events were boring.  Nevertheless, I thought that the baby outfits were darling.

I have not seen that at SSPX's parishes and I am in St. Marys, and have been to Winnoa, Post Falls and various California parishes. Each parish you will see dresses and styles of various kinds and sizes (and none Amish), unfortunately it tends to mostly short and tight.

Op here.  Prairie dress I mean