I mean the admission requirements to get a College education is just having magical talent of any kind and not being Dhar tainted or wanted by the Empire for crimes that can't be brushed under the rug, right? There is no admission test that the individual Colleges use to reject people?
Not everyone is suited for academia, but that isn't always apparent at the time of admission. And choosing a College isn't always based on a well informed decision, so they might well end up in one of the more bookish Colleges.
If those guys then don't ever qualify for Journeying, yet have an Arcane Mark and also can't be sent away to live normal lives with magic dampening jewelry (because they can't maintain a life style that you need to not be mugged) you end up with Perpetuals that can't be trusted with any job requiring independent decision making of any importance, but also are pretty much bound to their College for the rest of their lives.
None of the above means that their lives can't be productive though. They might be very fine janitors, or archivists, or cooks, or librarians, or childcare providers, or construction workers. But they'll have to do these things in the framework of their College and their College might not need someone doing that job in the Altdorf campus at that time.
So in my headcanon/imagination I thought that there might just be Perpetuals like that to spare. It's a far less deadly lifestyle that Journeying after all.
As for the Doylist problem of screen time, I wasn't even expecting that every staff member of WEBMAT would get some, just like the Greatswords and servants of Eagle Castle, the Stirland Watchmen, Julia Massif's staff and intelligence network and the various Rangers we interacted with didn't all get that.
Maybe my idea of how large WEBMAT will be once it is well established is very different from yours. Maybe six to ten Magisters don't need a larger staff to take care of stuff on a daily basis. But what I was imagining was essentially a mini-College structure with at least a few people needed who are willing to have day jobs maintaining the facilities and the clearance to be allowed to do so in the presence of Wizard work.