It seems that there are many studies that have compared mono and multilingual people. I wonder if similar studies have been conducted comparing a group of adults who are complacent with their mental development to a group who've made a conscious choice to actively pursue a deep understanding of a new subject that interests them.

I love learning new languages, but I understand that others may not. They shouldn't be restricted to endure learning another language just to achieve the cognitive advantages discussed in this article. There must exist other options that better suit their interests. Even if the newly acquired knowledge/skill must be relative to communication, there are more ways to express one's thoughts than written and verbal language.