Sorry don't mean to get off topic, but this point is salient to the OP's question. And to be clear, even 100% Dirt and Dirt/Rock combo features ALL require maintenance over time, period. Sure the interval may potentially be longer for dirt/rock, but that also depends on the local environment, weather, presence of livestock, usage volume and patterns, etc. For examples, there is nothing more satisfying to a cow then walking all over a pure dirt jump / berm. NOTHING!
I am of the persuasion that we need to break the mental crutch of most MTB'ers that perfect trails are perfect forever and if not, it was the builders fault. We need to promote maintenance as just as glorious as new mileage. I see rider after rider go around branches and sticks in the local trails, complain about hoof prints while not doing a single thing about them. While I not only interrupt my regular rides to remove debris and fill in holes, I also specifically don't ride some sessions and just walk and clean the lines (we have livestock for a good chunk of the year) so it wouldn't even matter if they were concrete.
Sorry a bit rambling, TLDR: All trails require maintenance and fairly routinely even if built with 100% dirt and Dirt/Rock combo in a perfectly "sustainable" style.