I saw how trail design changed in Nepal. I did my first trek there in 1983, and there were lots of vertical trails. I would guess that the average slope was 30 degrees in some areas. These trails covered traditional routes, so the local people were on them every day. To minimize erosion, they were lined with big flat rocks. So, we found ourselves stepping up and down these stone stairways. That got tedious, and it was hard on the knees.
By the time I returned to the Khumbu Region in 1997, the national park influence was heavier. They had some new trails specifically for foreign hikers, so they were rather longer and not so steep due to zigzags. The local people stayed more to the traditional trails, but there seemed to be room for everybody. The other thing gained with wider and more gradual trails was the room for yaks and dzos (beasts of burden), and that helped the tourism industry as well. The national parks there collect some healthy user fees, so that money is spent on trail maintenance and sustainable forestry.