1) I agree with the advantages of open source, but this project is commercial, so I have to find a way to make money. :(
2) Right. If I die, I will disappear. It wasn't rigid enough.
3) Gathering all the features in this app is not the goal. As for Todolist, it was not intended to be for general planning, but rather to assist in organizing knowledge. In context, one of the app's goals is to create specialized tools for organizing knowledge. Other tree-structured apps (e.g. Workflowy, Dynalist) are not specialized for organizing knowledge; instead they have a general purpose that can be used everywhere. I think this limits their functionality. LearnObit will implement specialized functions for organizing knowledge, not for general purposes.
4) I am thinking about the structure of knowledge that is not the tree structure. What I'm now going to implement further is for 1:1 pairs, which fit conventional SRSs like Anki. The app will deal with this by implementing a spreadsheet-like interface. I also understand that there is knowledge with graph structures, as you say. But I don't have an idea yet about what interface to deal with. Perhaps I will come up with ideas later.
5) For the tool you want, https://roamresearch.com/ might be good. This is a tool that combines Workflowy and Wiki.