Nom, yiddishkeit was definitely on the downturn before the Holocaust, in ways that most people don’t know. But that’s just it – it was a downturn. And the gedolim wrote a lot about it, including the trains that picked uo mechalelei shabbos in large jewish cities, women totally throwing away hair covering, and mixed dancing….but you’re not correct about mixed swimming. In all the letters from the chofetz chaim and others mentioning the sins of his day, he doesn’t mention it, nor does anyone else.
Also, you’re laboring under the assumption that yidden didn’t take yiddishkeit seriously until now; the haskalah ravaged European jewry, and even frum people were affected. But before and after that, we kept what the poskim wrote and what the gedolim taught. Do you think the exhortations of the chofetz chaim in all of his letters was made up ex nihili? Rav Moshes letter about tznius in America? The gedolim didn’t invent a new Torah; it’s quite the opposite; MO and its haskalah roots invented new, modern ideas and institutionalized sinfulness.
As it happens, I don’t know of any left wing rabbis who defended the mixed swimming. I think they just kept quiet about it.
And while European jewry was beset with problems, there were many strong communities where torah was paramount. My grandfather’s yeshiva town was one such place, until the last few years when the children joined hashomer hatzair and left Torah. Radin, mir, ponevezh, and many others were places where jews, as you say, took torah very seriously. Chasidim did too, and where did you get the idea that chasidim ever had a community issue with mixed… anything??? They were the only ones who as a community kept tznius, and keep it they did, with every chumra.