I reckon there's a place for those heavily groomed and sanitised trails. Especially close to the city or on the first couple of easy kilometres of any trail on a tourist route. As Rick says, you have to manage the erosion on heavily used tracks. When (we had in the past and are likely to have again next summer) you have coach loads of tourists wanting to get snaps of the view point from as far along the trail as the coach stop allows, you have to protect the track from the foot traffic.
But the other aspect is accessibility. We have a lot of bush tracks within the city and suburbs, and I think it's reasonable that a proportion of them, where the natural gradient allows, should be improved to allow access for disabled, elderly or infirm citizens.
Where I think it has gone wrong though, is on the long descent at the end of the Tongariro Crossing, or the similarly 'improved' section at the end of the Pouakai Crossing in Taranaki. The endless boardwalks are a huge scar on the landscape, are horrible to walk on and degrade quickly, needing frequent maintenance, which often involves helicoptering in supplies. These are supposed to be great walks in the mountains, surely there's a better way to harden the surface against erosion without sanitising the experience and ruining the view.