In those instances, there is no ecological consideration as the whole area basically gets wiped out.
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.
And here, I can assure you they are very much mono culture, evenly spaced, engineered species of trees, almost nothing else happens on the land in that time period. (generally it looks pretty similar to this https://a.rgbimg.com/users/p/pa/paparabbit/600/mwYGk8q.jpg) They do come and selectively thing at 5, 10, or 15 depending)
See the issue is, at least in the southern U.S. they clear cut nearly everything in the 1800s. There is very little old growth left.
P.S. As for mono culture timber, there is a reason almost no one in the US knows what a curant is. They (are/were) illegal in the US, because they are for a vector for a fungus that can wipe out timber pines.
I pity them. Real forest is something very different, than a wood farm.
With the recent droughts and dying out of the monoculture spruce, more and more mixed forest is coming back to europe. Idealism brought a little change, but the big drive was the huge economical loss of the timber industry that forced them to rethink how to do things.
There is a middle ground between the monoculture you describe and old growth. You can have an intact ecosystem with something every biologist would call "forest" even if you now and then strategically harvest some trees. We are almost 8bn people on this planet and we can't make do without lumber. All alternatives are much worse. Want to use plastic for everything? Steel? Concrete? We need to find a sustainable middle ground.
Linville Gorge being a notable exception: