Old Carthusian, very well said. But as the saying goes , you can lead a horse to water but you can't make it drink.
Bringing the discussion to the legal level. In the AIP I used to work with in my previous life , the rules are very clearly stated : in a controlled flight ,a pilot may not deviate from an ATC clearance unless he has received a new one or unless he is in an emergency situation requiring immediate action , in which case he must notify ATC to get an amended clearance.
I believe , but not sure, that is even a standard EASA wording.
We can debate as long as you wish, those are the rules we both , controllers and pilots have to work and live with in that particular airspace . I very much doubt that any airline will accept/allow that one of his crews deviate from this, because if they do and something goes wrong, the costs of the mess will fall on the airline insurance shoulders.