In some sense it was good, and basically help shape modern Denmark, but it's just not needed anymore, and has come at the cost of wildlife, native plants and sea creatures. It didn't start out like that, but when you add modern intensive farming on top of killing of most of your nature areas, then things starts to go very wrong. Denmark has almost nothing of it's original nature left.
exactly, and we're talking about Denmark, after all
Source: I’ve planted thousands of trees.
I don't remember the details, but I believe it goes something like farm -> heath -> shrubland -> young forest -> mature forest, where each phase has a unique ecosystem of both plant species and animal life.
In an extremely heavily cultivated landscape like Denmark (seriously, look at a satellite photo), converting farmland back into forest is a multi-decade project requiring constant maintenance. Converting farmland into marshland (which is the "original" stone-age landscape in many areas) is a multi-century project.
Just like it was a multi-century project to convert it into farmland, by the way. Europe has been cultivated for millennia.