I've always interpreted the 'scrum master' role as the person on the team that is both the process expert and senior enough to be able to generally coach others and help unblocking issues. Ie. one of the senior devs on the team that has been interested enough to learn the scrum agile process, but who takes on the role can change depending on circumstances.
But obviously this means it's not a dedicated role and so a missed 'opportunity' to create more "manager of something" roles, especially for project managers who feel threatened.
More responsibility for the same pay, corralling people, and inevitably adding their workload to yours? Because SME/mentor < sr dev when it comes time to rightsize the CxOs gambling losses.
Not surprising sr devs weren't jumping on this whole hog.
But obviously this means it's not a dedicated role and so a missed 'opportunity' to create more "manager of something" roles, especially for project managers who feel threatened.