Thank god most are not maintained, getting fed up of 20km pump tracks.
Thing is, if a 20km pump track as you say wears out, you don’t get a nice interesting naturalish track, you just get a pump track that doesn’t work. Some trails thrive when left alone, some just die. The trick is appropriate maintenance.
Also, you have to remember the trail lifecycle. A new trail will generally be smooth and wide and often grey. Roots and rocks are buried, etc. It looks like a BMX track. But! That’s just because it’s not finished. Building it basically lays the foundations, then tyre pressure turns it into a trail over time. The dominant, narrower line gets ridden in, vegetation encroaches, and the surface evolves allowing the buried features to emerge. But the strong foundation remains- the wider stance for the trail, the solid material around the roots and rocks etc to stop holes forming.
It’s hard to build a durable trail with a naturalish surface but there are many trails that started out looking like a hardpack path that now feel great. Ironically I have heard people say, while we build a new bit, that we shouldn’t do it this way, we should do it like section X- when section X was actually built in exactly the same way, a year ago.
Put it a simpler way- armoured trails tend to be boring when new, lovely when worn in, annoying when worn out. The exception is berms-n-jumps because they’re fun straight away, but they go off faster. The trick is to make the worn-in phase last as long as possible, with good building and design and thoughtful maintenance. But this is hard!