It's true that most forests today are not old growth anymore. They have been cut (usually clear cut) and then either replanted or just naturally reseeded and are now essentially waiting for the next cut. There are some areas where they are monocultural tree farms with not much else growing, but forest industry starts to realize that that's not the most efficient anyway. Most forests today, even those used for lumber (which are almost all) are integrated ecosystems. That's a great benefit to nature.
Clear cutting is quite bad for the surrounding ecosystem. Habitat loss, soil erosion, it's a pretty traumatic event for a forest. (Some shrubs really thrive for a few years after a clear cut though. In Sweden that's when you get to pick tons of wild raspberries, for example.)
An alternative is selective cutting where you only cut the oldest trees and leave everything intact. For that, such a legged harvester would be much better. The traditional method is to use horses, but that's too slow for modern forest industry.