Excerpt from BBC 6 Minute Grammar:
If the weather gets worse, our plane could take off late. We might not take off at all!
Now that’s all fine, but if you say: We couldn’t take off – you’re talking about an impossible situation in the past, not an uncertain situation in the future. So you can’t use couldn’t in our airport example.
So they say that the following sentence is incorrect:
If the weather gets worse, our plane could not take off.
But is it true? Perhaps the weather is already bad and if it gets worse I am certain that it would be impossible to take off (i.e. we couldn't take off).
In the same programme they also say:
CORRECT: The traffic is getting worse, so I may not be home on time.
WRONG: The traffic is getting worse, so I could not be home on time.
But perhaps I can see the situation and I am certain that it would be impossible to get home on time (i.e. I could not be home on time).
And the last one from the same unit of the course:
We don’t use couldn’t in the same way as might not/mightn’t/may not. ‘Couldn’t’ means that something is impossible.
WRONG: I think there couldn’t be any trains today, so I’m going by bus.
CORRECT: I think there mightn’t be any trains today, so I’m going by bus.
Well, probably I can see on the timetable that the last train has already left and there won't be any trains today. So here it's impossible to be any trains today (i.e. there couldn’t be any trains today).
So my general question is: what is wrong with using couldn't for future impossibility in these examples?