This can't possibly be true. 65% of Americans are living paycheck to paycheck. They definitely aren't in a position to get a Sauna.
And besides, just because people can afford to do something, doesn't mean that less wealthy people do. Almost anybody could afford to golf, but that doesn't mean golf doesn't skew wealthy if you're looking at the people who actually play. If you looked at how long people who golf live vs people who don't, I'd be willing to bet that golfers live longer. I'm not about to suggest that golf is what is keeping them alive though.
Here at home I belong to a gym for $55/ month and sauna there almost daily, and in prior home in Green Bay, each YMCA had a sauna included with membership. Mostly northern cultures and many southern have had something similar for thousands of years. The Oneida tribe outside Green Bay has regular sweat lodge ceremonies, same basic thing. I won’t enumerate the benefits of a true deep heating sauna but it is deeply meditative for me. This is one of the healthy ways to relax that is often available if sought out. Even during a recent trip to Orlando, I was able to sauna at one of the Y’s. My dad (86 yo) saunas at his health club near his home in SC.
I have never heard saunas described as something outside normal economic lifestyles.
And if there isn't fuel, they can simply relax by the pool.
Let them eat cake.
While there, I became friends with an old Finnish couple about 10 miles east of town near the Lake Superior shores. They explained that until just a few years ago (at that time), people had outhouses, and a sauna for relaxing, bathing and socializing. They did not eat cake. (They ate pasties :-) )