It is not that they want me to spend work time on it. They want me to transfer ownership of the project to them. That's the part that I do not wish to do.

The way things work at this place, I'll probably end up maintaining the product until I leave my job.

The reason for not transferring over is that once I do that, they will essentially not let me create features that I think can be done better using my product, because they exist in some other product.