I think you should orient yourself on already released titles by Colossal Order, like Cities in Motion (2) and Cities Skylines. They usually don't just add another variation of the same vehicle type if there is no valid reason whenever a third party could create it just fine out of the box.
They only create new assets if they need it for a new feature (free update or DLC, like adding a monorail network) or if they have some spare time left (like on Cities Skylines 1's farewell update https://forum.paradoxplaza.com/forum/developer-diary/hotels-retreats-dev-diary-2-free-update.1581200/).
CO basically focuses on providing a platform which is easy to expand through all different ways like DLCs, feature updates, content creator packs, free regional packs (which also may include vehicle variations developed by third party creators from within the community) or mods.
To me it's pretty fine this way as all such asset mods and packs will be available across all platforms when the time has come. If CO would start now on developing x variants for the same ai type with the same stats just for the looks they won't finish that anyway until mod support is implemented because they got all hands full to do with other stuff already .
As I also am working on assets to be released I at least can tell you that I don't feel like doing COs "job". Instead I love the opportunity to be able to create what I wish to see in the game on my own and share the results with thousands of other people around the globe - and I know many other creators which think the same way, so don't worry about "us".