Let me be more specific about what I mean by giving airlines the wrong incentive and the likely effects of the proposed new thresholds being contradictory with the asserted aim of getting passengers back home 'as soon as possible'.

Example 1: You are flying, say UA Airways from LHR to PHL. The plane has landed at LHR, been inspected and a mechanical has been found which requires the airline to change a part on the plane. The part is available at LHR from their sub-contracted technical support (please note I have no idea whether US sub-contracts maintenance at LHR etc it is just a random and arbitrary example) but they also have it in stock at their home maintenance unit in PHL so it is cheaper for them to bring it over than to have it replaced locally. Before, the airline would have had a clear incentive to pay that little bit extra and have the plane fixed as soon as possible to avoid paying compensation. Now, they can just choose to ask for the parts to arrive from PHL, as the distance flown is over 6000 km (a mere 3200 nautical miles), they have 12 hours to find the cheapest maintenance option for them at the risk of further delaying passengers by several hours.

Example 2: Again, you are flying with DL from LHR to BOS. The airline decides that they need your plane to fly a lot of overbooked passengers to Atlanta while your flight only has 80 passengers and don't want to have to pay them denied boarding compensation. They can rebook you on the BA's LHR-BOS flights which have plenty of space or bring a small 757 that needs to fly back from Europe to BOS anyway after being through maintenance and which can fly from LHR in 11 hours. Before, the airline would have had a strong incentive to put you on BA to avoid paying delay compensation after 6 hours. Now, if they find that this is cheaper, they can just make you wait for 11 hours for that 757 and there is simply nothing you can do about it.

Of course, I know some people will say "oh, but it is not in the airline's commercial interest to make passengers wait longer than needed" but if that was a certain enough assumption, then we might as well not have any consumer protection at all and let every company do whatever they want with the certainty that 'doing what's right is in their commercial interest'. Personally I simply don't buy it, and if you think of the people flying FR or, say, a charter airline transatlantic, I'm sure many will share my concern. And as for saying 'well then don't fly charter/Ryanair', we have to recognise that consumer protection is precisely here to defend the interest of those who are poorer and more vulnerable and may therefore not have the luxury to fly on a fully flex F ticket and carry a super duper travel insurance.

Finally, bear in mind that a 6000 km long by plane will typically take the same time to travel as many a London-Scotland rail journey. Would we consider it fine to have the train companies only compensating delays on such routes if they exceed 12 hours?

It is in this sense that I believe that the proposed changes create the wrong incentives for airlines and contrary to claims, won't lead to passengers being brought home 'as soon as possible' more effectively. As mentioned by others, there are other ways of ensuring fairness, for example, increasing the period where only half the statutory compensation is due, thus creating a 'scale' or increasing the delay franchise for shorter flights but not for longer ones. But I can only restate what I said before - to me, 12 hours before any compensation kicks in is simply way too long.