I think a lot of trails get closed because of "bad apples" who can't behave or follow instructions - those guys ruin it for everybody. There are also trails that get closed because of poor design where water can't properly drain and huge muddy ruts form or the topsoil washes out from the trail leaving it nearly impassible. The resulting erosion then fouls the streams and creeks and rivers down stream, affecting fisheries and overall water quality. A lot of water crossings I've seen can also become mud bogs (from the traffic) that also foul the water.
I'm guessing the USFS may be more agreeable if the local jeep clubs step up to offer their volunteer labor to make needed trail repairs and improvements. They can't possibly keep up with the necessary maintenance with shrinking budgets and increased costs of fighting wildfire and whatnot. I know that at least down here in Colorado, the USFS is very happy to have volunteer groups step up to share this load. We have a hiking trail here that is opening this month after being closed for 5 years from a fire followed by floods. The trail was destroyed and only is opening because of several local volunteer groups who rebuilt it.
That pretty much sums up the situation all across the west. With the cost of fighting fires draining all their maintenance, enforcement, and virtually everything other budget, they no longer can maintain trails or even the signage. And as in the quote, we are most definitely our worst enemy. Most mountain trails we frequent are boulder fields (rock gardens). They were not that way when built but are now that way because one to several feet of fine material has been eroded from them. And as above, a large percentage of that ends up in streams. And then there is the trash. Why do motorized users think leaving trash everywhere is ok? Does it cost too much in gas consumption to pack out empty what you brought in full? I always end up with a trash bag full of trash some clueless idiot left behind.
There are all sorts of places around here I can no longer access with a motorized vehicle that I used to Jeep to. Comparing those sites now closed for nearly 50 years to those that remain open is a rather eye opening experience. I've been backpacking since I was 12, 62 years ago and counting. I learned to drive in a CJ2A a couple of years before then and have been piloting at least one ever since. In my opinion, and sorry if you don't agree, motorized backcountry use is the most destructive of any forest use other than large scale mining. That said, I still enjoy those trails.
Especially so if driving the Willys.