This "append-only" style also seems like it could lead to new code being added just to "undo" what the existing code is doing, which is obviously waste.

Reminds me of a process I saw at work years ago. An employee was given an Excel macro to run on a weekly spreadsheet file. The instructions included "Unhide cells D through F". It turned out that the macro had code in it to hide cells D through F, but the employee didn't know how to just remove that code from the macro, so they had to manually undo something the code did, every single time they ran it.