It doesn't imply you'll have fuel to another port if your destination suddenly requires an alternate due to un-forecast changes, however if you have fuel to another port at the time it deteriorates, your obliged to apply the altn requirement and divert, or if it goes sour you'll answer for it.
I agree 100% with everything you've said in the above quote. That is exactly my understanding of the regs/company manuals.
The inflight quote ensures you don't continue on to destination after departure if your fuel level reduces to the level below what your min arrival fuel is pre departure.
I disagree with the above.
Your min arrival fuel pre departure is not relevant. None of the regs or company manuals suggest that the PinC is responsible for comparing destination fuel with a figure calculated pre departure. But they all suggest that he/she is responsible for ensuring that, based on the latest available forecasts, they have fuel to an airfield that has wx above alternate minima. QF goes so far as to point out that that airfield can be Destination, an Alternate, or any other suitable airport to which an Enroute diversion can be made.
At some point after you leave behind that last suitable and your destination wx goes below altn min you will obviously not have altn fuel. Or if your last suitable enroute goes to altn as well.
Correct. And in that circumstance the QF manual says
It is recognised that there are occasions when a flight may pass DPA with the required fuel on board and a subsequent deterioration in forecast weather may then result in the minimum mandatory requirements above not being met.
That is life, and at that stage there is precious little you can do about it, but the manuals have been doing their damnedest to prevent that circumstance from occurring by instructing the Captain to make decisions that ensure the 'minimum mandatory requirements ' are met at DPA.
73Q, I don't think you're a High Capacity pilot or you'd know that you're quoting is what you'll find in every airlines Fuel Policy, and you're taking it out of context!
What's a high capacity pilot?