But why does that have to be the case?
I argued about this quite a time, being of the opinion that it hasn't. I think it is just a bit tricky to implement.
Surely corona is just constantly comparing its pass/noise level number to whatever that spinner value is to determine when to stop.
It just doesn't. It constantly compares to internal values which were set at render start according to the spinner values (and other settings in the render setup UI) as far as I know. I can understand that most of those settings have to be static and cannot be changed after being initialized. But stop conditions could be handled dynamically imho. If you take for example noise level - it's dynamically calculated anyway and leads to a render stop when reached. So why should the value set in the UI not been compared/refreshed after every pass? And even when thinking about DR I see no reason because DR slaves just silently render passes until they get message from master to stop.
If inconsistency is the argument then being able to adjust the tonemapping controls mid render is just as dangerous imo.
I think VFB / tone mapping runs in a completely different context. Anyway...
Ideally id like to see a way to do it without having to stop and resume the render as you pointed out in that linked thread.
+1 (again)
Good Luck