For me, the realism is definitely there where units get stood down en route or on scene. In fire dispatch, we often send trucks responding to a scene back to their station when the call is of a minor nature, but some calls, it's only determined minor after a few minutes, so units are being released before the call is completed. This happens for expansion down missions, where a Rollover turns into a regular MVA or something, where units may be on scene and not be needed any more. It's much quicker to take them down in one fell swoop than one at a time. As more expansions are added, making calls more and more minor, this makes sense to add.
Another call that comes to mind where units being released could be beneficial is the Active shooter. Once the police are done, that's when the patients appear, but this means that I've got one helicopter and 4 SWAT units (maybe more) tied up waiting for the paramedics to clear the patients.
A different solution would be a cancel select option, similar to how we dispatch units. Select the units you want cancelled (en route or on scene) and cancel them, so we'd have a cancel all, cancel ambulances and cancel selected buttons. That's the best solution in my head
So you've picked up on cancelling on route, which is realistic and through the current system is perfectly doable. I haven't, however, found a situation where I've needed to bulk downgrade, only a few specific units at most, there is no mission that significantly downgrades in the game.
Downgrading resources from a scene is not realistic. When units arrive on scene they are actively dealing with the situation in front of them, and until the task is finished or suitable resources can replace them, there is no standing down. Using your example, at an active shooter scene, the police will actually be around for a lot longer than the paramedics. When a fire engine turns up at an MVA, they may well have to do some serious unpacking to carry out an extraction. Until the patient is in the back of an ambulance, they have to wait. You cannot leave a patient by the side of the road waiting for the ambulance to come along.
I hope this has put this into a different perspective and helps understand why I have this position on the topic.