The fact that terrain making requires non-Workbench steps is 100% fine. However, I think Satmap generation in particular should be a World Editor feature for the following reasons:

1. The official terrain making guide currently instructs you to use textures from the game, however there is no way to export these, meaning you have to inconveniently take and crop screenshots. This clearly isn't an elegant solution.
2. There is already a similar feature for ingame map rasterization, meaning the technological foundations for such a tool already exist.
3. Satmaps should be generated from materials, not just textures. When using just the texture approach, you inevitably run into issues: That middle texture might be colored differently by the material, it's always scaled in some way by the material and has additional PBR properties which are not included in just the texture. Trying to manually adjust for all these is impractial, but a Workbench script could take these into account fairly easily.