I have a theory, that revolve around the idea that Thanos failed to snap correctly.
Yes he failed !
How did he failed ?
Thanos promised that everyone will die in an instant, without suffering.
Was it the case ? No !
Not only not everyone died at the same time (Peter died before Strange for example), but more importantly, the death was not instantaneous.
And this does not fit the mercy killing.
Was Peter suffering physical or psychological, the jury is still out. It does not change the fact that seeing yourself slowly fading away can be traumatizing, even if only for a few seconds.
But more importantly, it gave enough time to Fury to call Captain Marvel.
But why did Thanos failed ?
May be it's related to the fact that he had a big weapon embedded in his chest at the moment of the snap, it tends to impede concentration.
If they didn't fight, even if only for a few minutes, all the event chain would have changed, and Thanos would have been able to snap his fingers while fully concentrated.
What it would have changed ?
Well, If I have full power over mind, soul and time, and wanted to kill half of the universe, I would not only kill people. I would retcon it so that no-one will remember what happened. So that no one will suffer even from the loss of a loved one.
So no one will even know that I have killed half the universe (So no-one will try to revert it).
But it will be difficult, I will need to concentrate hard.
I don't remember if the Gauntlet was broken before the snap, because of Thor attack, giving another reason why it did not work exactly as intended, or after, confirming that Thanos did not use it properly.
But in both case, it confirm that it's Thor timely intervention gave a chance of saving the day.