Ok so hear me out...
Why do people drive diesels?
1. There are those that think they are being environmentally sensitive as the emissions are less, and therefore less harmful.
2. There are those that drive alot of miles each year (18k +) so driving a diesel is actually financially the more sensible option, so long as it's not in a large diesel engine (2.0 max), as you need to be getting 40+mpg to make this work.
3. And there are those that think that as they are getting a better MPG figure and therefore more miles per tank, it must be cheaper than running a petrol car, which is completely wrong, due to the extra base cost of buying the fuel.
If you do c10k miles per year the actual difference between a diesel at c43mpg and a petrol car at c30mpg is only about £1 per day cheaper (on the current difference between petrol and diesel ~ 12p/ltr). Does that sort of difference make it worthwhile? I don't think so.
What's the long term scenario? Diesel costs more to produce so the liklihood is that it'll always cost more than petrol. Both diesel and petrol engines are (as you would expect) becoming ever more increasingly technologically advanced so, emissions and fuel efficiency should always be improving. The recent introduction of twin turbo's will help even more on both petrol and diesel motors.
I would imagine most people fall in Category 3 above.
What's the future for high performance petrol cars getting c15-20mpg, like our S4/RS4/S6/RS6/S8 & R8 models ? Soon it will come down to those that don't really do a great deal of mileage therefore it doesn't cost a massive amount, and those that have so much disposable income that they just don't care. Are we going to see those models all be given performance diesel engines so as to keep the S and RS marquee alive, as in the new R8 V12 diesel (but maybe a tuned 3.0 unit)?
This is meant as a semi-serious discussion, so please treat it as such...