I guess it depends on the servers. I'm in academic/research computing and single-user systems are the anomaly. Part of it is having access to beefier systems for smaller slices of time, but most of it is being able to share data and collaboration between users.
If you're only used to cloud VMs that are setup for a single user or service, I guess your views would be different.
This is overwhelmingly the view for business and personal users. Settings like what you described are very rare nowadays.
No corporate IT department is timesharing users on a mainframe. It's just baremetal laptops or VMs on Windows with networked mountpoints.
When you have one OS that is used on devices from phones, to laptops, to servers, to HPC clusters, you're going to have this friction. Could Linux operate in a single-user mode? Of course. But does that really make sense for the other use-cases?
Yes. First, we use user level container systems like apptainer/singularity, and these containers run under the user itself.
This is also same for non academic HPC systems.
From schedulers to accounting, everything is done at user level, and we have many, many users.
It won’t change anytime soon.
I understand academia has lots of different accounts.
Hence all containers are isolated from each other, not only at process level, but at user + cgroup level too.
Apptainer: https://apptainer.org
Second, there are the jobs users submit. These are often executed on separate nodes and the usage is managed. Here you have both user and cgroup limits in place. The cgroups make sure that the jobs on have the required resources. The user authentication makes sure that the job can read/write data as the user. This was the user can work with their data on the interactive nodes.
So the two different systems have different rationales, and both are needed. It all depends on the context.
The primary point of user-authentication is that we need to be able to read/write data and programs. So you have to have a user-level authentication mechanism someplace to be able to read and write data. cgroups are used primarily for restricting resources, so those two sets of restrictions are largely orthogonal to each other.
Second, user-authentication is almost always backed (at least on interactive nodes) by an LDAP or some other networked mechanism, so I'm not sure what "cloud" or "k8s" really adds here.
If you're trying to say that we should just run HPC jobs in the cloud, that's an option. It's not necessarily a great option from a long-term budget perspective, but it's an option.