I want to know for what reason we cannot use “what when"

What if defined as a phrase means:

What would result if ….? (Oxford)

‘What if nobody shows up?’

The differences between if and when is not the question, but if we say what when meaning:

What result when….?

can we say:

What when nobody shows up?

and if not, why?

Solution 1:

The difference between when and if seems important in this construction. Where if means that something may occur, when implies it probably will occur at some moment in the future.

That means that what when nobody shows up... sounds a bit strange, because it seems to imply that we already know that nobody will show up - which is not really a well defined event, because it presumably is equal to the current state of affairs.

On the other hand, when you organise a garden party and the dark clouds forming in the sky make it quite clear it will start raining at some point, you could ask yourself what when it starts raining?.