Honestly, I can't say that it fails in any way like that. It has power, comfort, etc. in abundance, and as I said at the start, it has been a good friend.
There are probably 2 or 3 main reasons driving me in considering this.
Firstly, its pretty clear that the government plan to use tax policy to encourage us to change our ways as regards pollution, climate change, etc. The NOx Levy is probably only the first step. Road tax may well be the next and then further scrutiny of CO2 outputs. Given the fines that will be levied on manufacturers for every car they make if their fleet average CO2 figure exceeds 95g/km I think its fair to expect that cars that were recently considered as decent CO2 performers will soon enough be social pariahs, and taxed accordingly. So, a 272bhp 3.0l that is pretty much a gas guzzler by current standards for simple commute and chores may be like using a sledgehammer to crack a nut. I have other cars that fulfill the social pariah role so one that doesn't reach in to the wallet quite so skillfully mightn't be a bad thing either.
Secondly, I've had the 530i for more than 4 years and the mileage passed 130k recently. That doesn't mean its clapped out, quite the contrary, but both of those are fairly high figures by my standards and the itch is there.
Thirdly, climate change isn't just about being hit by tax policy changes. If I can do something that reduces the overall impact I create then maybe its just time to actually do it. Its a small step and won't save the planet, but its something that I can do, without having to wear sack cloth and ashes, and if I wait for everyone else to do something before I do, then we're probably doomed