Listen, I get it. At surface level, it's easy to see similarities between a Scrum Master and a traditional manager. Both are there to bring out the best in developers. Both accountabilities keep an eye on the big picture and help the team succeed. It's a common mistake for upper management to make, especially when looking at the budget and trying to cut costs wherever you can. If you're looking for a team of un-motivated developers who will do work at a snail's pace, have narrow visions for the products they are working on and are secretly looking for other jobs that will better fulfill them, then sure - make their manager do the Scrum Mastering. But if you want a team of dedicated, talented developers who will go the extra mile to make the product the best that it can be, you better keep those roles separate.