Fair points, I tend to conflate agile and scrum since they seem to be coupled together. I get that the dedicated part isn't a requirement, it just seems to be part of most implementations I see. I guess my problem is more with people's implementations.
That's not a part about Agile, but about Scrum.
Also, the “dedicated” part isn't even essential to Scrum.
> We had agile coaches at my company recently and it was one of the more painful experiences of my career. They would try to add [...]
That's a different issue than a dedicated ScrumMaster role, and anti-Agile, as team ownership of process is central to Agile. Consultants external to the team (whether internal to the org or external there, too) trying to impose process is the antithesis of Agile. There might be a role for coaching in Agile, but it certainly isn't process advocacy.