In addition to that very good point about controlling for socioeconomic factors, the article says a single museum or concert per year makes a difference. Most cities have free community concerts (some even have free opera performances!) and museums that are either free, pay-what-you-want, or at least have specific days/times during which they are free or at a significantly reduced cost. Many libraries (which are free) provide free museum passes to card holders. In fact, the article quotes a museum worker who works at a free art museum in Baltimore.
If you actually read the article you would also read that educators are excited about this study because it provides evidence that the arts should be made more accessible financially – by restoring arts programs in the public schools, for example.
They controlled for socioeconomic factors though! The people who conducted this study knew that people with lots of money to attend the opera were also more likely to be able to afford basic necessities, so they controlled for it in their analysis. The fun thing about statistics is that you can control for different confounding factors so you can look at the effects of one independent variable (opera or whatever) on the dependent variable (mortality). Part of being critical of potential biases is actually reading the article and knowing what to look for.
this week in I Am Very Smart: having enough money to go to the opera, museums and concerts correlates with having enough money for food, shelter and basic health needs
