This issue has been on my feature list for a while, too.
I would like to have a record that I completed my tasks, even if they are repeated daily (did I really wash the dishes today? 😉). In other outliner/task apps that I’ve used, there is often an option to either leave a completed version of the task in place or just erase the completed one like Legend does now.
If making this an either/or option is too complicated, I would like to just have the completed versions stay on my calendar/list which is the opposite of what is happening now. At least that way I have a record that I did something today!