You’re trying to incorrectly put words in scientist’s mouths here.

They did not say that a person should not have children.

They merely said that on average, people who don’t have children are happier.

If people could accidentally find themselves trapped in a heavy metal concert, (just like people accidentally find themselves stuck being parents), you’d find a similar conclusion - people who don’t go to heavy metal concerts are happier. But it turns out that concerts are elective, so the effect is unlikely to be present in real life for concerts.

This has nothing to do with the “goodness“ of concerts or parenthood - both of them are awesome when the people doing them chose to do it!