Systems administration and operations are tightly scoped. DevOps is, in my opinion, an umbrella term representing all non-core engineering work: servers, build pipelines, on-call response, infrastructure, and more.
The tricky part is that a lot of these areas are relatively new, and the older ones like server admin have changed dramatically over the past two decades.
DevOps hasn't failed. DevOps is in its infancy and is going through technical, strategic, and philosophical growing pains.
DevOps has pivoted I think from the more original goal of having Devs do a lot of Ops things. This isn't really a viable solution as almost every compliance system mandates the people writing code and the people deploying/running systems are different.
What we have now is Ops that use things like source control. Write scripts that are more modular, reusable and composable. A new application can have cloud resources created and allocated in a day rather than a month of tickets to several other teams. This is also granting visibility to things that Ops may have been doing, but it was scripts on a PC or build server that only Ops had access to.
The tricky part is that a lot of these areas are relatively new, and the older ones like server admin have changed dramatically over the past two decades.
DevOps hasn't failed. DevOps is in its infancy and is going through technical, strategic, and philosophical growing pains.