I think the main thing is that breaking a significantly changed file into an add/delete pair feels extremely confusing for a blame operation. I suppose conceptually if the lines in the rewritten file originated in a different file then it might be useful to track them back, but the confusion of saying that file A existed prior to this commit and yet we're going to use the history of file B instead is too risky. Also, the break threshold would have to be much higher than the default for this to not cause confusion. I think using simple renames is probably best.