The problem is you're trying to write a one-size-fits all solution for an application. Not just a library that loads a model (like Assimp) but also gets it on the screen, so you're locking in users into how things are formatted and function. Basically you're offering a platform, which can be problematic if a user wants to do something different. And you aren't going to get buyers unless if there are a lot of features that sets it apart from free/open source competitors.
I think you'd do better if you wrote plugins (for profit) for an already established and mature platform. E.g. write something useful for Unity and sell that.