Of course you should always consider changing circumstances. You should also look ahead and try to predict what might change. How likely is it the situation goes from basically controllable to basically uncontrollable? How quickly will that happen? How much will you be able to affect that?
I suggest that there are several potential ways in which an open door can go from "basically not so bad" to "very immediately hazardous" very quickly. And the way to prevent or reduce the risk of all of those things is stopping the train.
Comparing the two real life situations in which I pulled the cord. I don't know if I did the right thing. But, in the heat of the moment, that is what I did.
In the first (Green Park), when the doors failed to open at Bond Street it seemed of little consequence. However, when it happened again at Green Park, I saw people started using the connecting door into the next car. I was aware that this would be potentially dangerous if the train started to move and decided to get in a position to pull the handle. Even then I waited until the doors closed in the next car and heard the brakes release, and saw there were people still crossing over the gap, before pulling the handle.
Contrariwise, in the second case (Waterloo), I pulled the handle immediately I realised the other passenger was having an epileptic fit (the onset of which seemed to coincide with the train starting to move), to stop the train whilst it was still partly in the platform, because I realised that if another passenger were to pull the handle between stops we, and in particular the patient, could be stranded on the viaduct for a considerable time. Had we already cleared the platform I would have tried to stop anyone else pulling it until we reached the next station.
I was surprised that, in neither case, none of the staff enquired as to who had pulled the handle or why although, to be fair, at least in the second case the reason was fairly obvious.