This seems to be a low effort, high reward way of doing things. What extra benefit does Zettelkasten bring and is it worth the additional effort?
EDIT: Another consideration is that you don't want the time you dedicate to sorting a productivity system to be especially big. You want to make stuff. The big txt file doesn't take up a lot of time to organise and allows you to spend time doing good things.
[1] https://news.ycombinator.com/item?id=22276184 (entitled 'My productivity app for the past 12 years has been a single .txt file')