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.
As a Scrum Master, it is frustrating to see so many people misunderstand the accountability. You are not a task master. You are not micromanager. The main descriptor of the Scrum Master accountability is to be a servant leader, which means serving the needs of the Scrum team and leading them to success by guiding them to the best behaviors using encouragement and introspection. Unfortunately, some Scrum Masters think that as long as the end results of ensuring projects are delivered on time, it doesn't matter about having good working relationships with the development team. This is wrong. If you have this mentality, you are yourself an impediment to your team's success.
Scrum and Waterfall are two of the main methodologies used in project management, and they each have their own unique characteristics and approaches to managing projects. The Scrum Master and Project Manager role from the Waterfall methodology are two roles that are critical to the success of a project, but they have different responsibilities and focuses.