Parable for an impossible idea or business plan

Solution 1:

An established expression for this is pie in the sky:

From Merriam-Webster:

: an unrealistic enterprise or prospect of prosperity

And from Lexico (Oxford), which provides example sentences:

Used to describe or refer to something that is pleasant to contemplate but is very unlikely to be realized.

‘It's probably pie in the sky to say we could unionize them, but that's what I'd like to see.’
‘We are not talking pie in the sky, we are talking about clear correlations which will help deliver a healthier Scotland.’
‘We are still in the preliminary stages but we are seriously interested - it is not pie in the sky.’

So, to rephrase the example dialogue in the question:

Engineer: But the speed-of-light limit to Australia is 67ms, so this is just pie in the sky.

Solution 2:

You can use the expression castles in the air meaning:

plans that have very little chance of happening.

Cambridge Dictionary)