A single .txt file can work as a to-do-list and a what-done list but it really isn't a knowledgebase like Obsidian or Roam. It is more comparable to a To Do List app like Microsoft To-Do or Amazing Marvin.
It wouldn't work as a knowledgebase because a knowledgebase has copious amounts of text about a topic which wouldn't fit neatly in the by day subdivision.
Although, I don't use Roam Research, I think the single .txt file approach also has similarities with the Roam style of using Daily Notes and Outlines.
Roam Research is an outliner which emphasizes not using paragraphs of text but instead just using bullet points. His single .txt file approach is also using this outline and bullet-style approach and I think it is an underrated aspect of why his approach works.
Using an outline and bullet points helps a lot, to quickly see the forest and the trees; I've only recently realized how powerful this approach is and so I think it's worth mentioning.
On a personal note, I've operated for years without a to do list app and so I can easily imagine someone using a single .txt file and being way more productive with it. But I highly recommend anyone to try Microsoft To-Do or Amazing Marvin or the single .txt approach since I agree that it is better than going without.