I think being smart universally trumps being less smart - but the definition of smart needs to be more encompassing. For example take the stereotype of a brilliant man or woman who neglected family for career, and are forever single and childless forever -- while someone less brilliant just "ends up" with a family and ultimately more fulfilled in life. Superficially this is an example of an average person being happi…
As a single and childless person, I don't like the implication that those are marks of a less fulfilling life. I know some people married with kids who seem very happy and fulfilled, and some profoundly miserable and trapped. I may be missing out on some good things, but I'm also avoiding some extreme misery, and generally do feel content and fulfilled.
The contrast you provided between those who are happy with family and those two seem miserable is in line with what I am saying: it's both 'be smart about what you persue' and 'be smart about how you persue it.' So likely the first group was 'smarter' about who they chose as a spouse and how they set up the life with them than the second group.
So maybe we can score the miserable group as a -1, the happy group as a +1 and your avoidance strategy as in-between: 0.
So you can look at your strategy as good because it avoids the extreme misery but it also denies a shot at extreme happiness. And then if one were to go that potentially high risk/high reward path, their actual outcome (are they happy or miserable) is a function of how smartly they persue that endeavor.