I can see a clear need for custom routes with included sessions, rolling stock and assets, etc for mobile and gaming platforms (XBox, Nintendo, etc) but where is your evidence for claiming that N3V seem to be moving towards this for desktop platforms and bypassing the DLS? Certainly the DLC routes and sessions for desktops are customised this way but they provide a particular product for users who do not want to spend the time/effort creating their own routes and sessions, or lack the necessary skills to do so.
Products evolve over time and that almost invariably means that they become more complex. You have to ask why have N3V introduced more complex meshes, PBR textures for object and ground surfaces, LODs, imposters (and the creation of imposters seems to be done automatically "in game"), etc? The answer is consumer demand for better graphics, more realism and improved performance. That usually means that many of the older simpler assets that worked well in the earlier Trainz versions are no longer suitable or acceptable.
If the latest versions of Trainz are demanding that assets must meet higher standards and are more difficult for the average hobbyist user to create then there is a solution - stay with the older less demanding Trainz versions.
My thoughts.