I think the whole idea of who would buy a diesel soft top is difficult to understand for anyone with even the smallest interest in cars. Allow me a brief explanation:
Most people do not have the foggiest idea about cars, and will buy one purely for how it looks and what colour it is available in. People will buy a red car on the basis that someone they know used to have a car in the same colour of red and it was either "sporty" or gave them no reliability issues. All models of cars are exactly the same, across the range.
Where I work, there are people who will go out and buy a brand spanking new car to avoid the inherant dangers and costs associated with MOT-ing their 3 year old car. This 3 year old car is immaculate, you understand - fastidiously maintained by the main dealer, and all the service stamps in order. No expense will have been spared, but after 3 years and 15000 miles, the driver can 'feel' the car almost on it's last legs and would like to get shot before something bad happens.
When the type of person outlined above decides, on a nice day, to go and view some cars at the Audi showroom, they are a salesman's dream. They have a budget of 'x', which naturally means the only car available to them is an Audi A4. There will be a vastly below-par part-ex offer on their old car, but that's fine, because what the salesman doesn't know is that on it's first MOT/service it might need a new front tyre and perhaps the air filter changing. Haha! Suckers!
They'll want a red one, of course, because red is better than blue. They'll want to spend thousands on manufacturer-fitted sat-nav because they've read horror stories of cars getting broken into due to the suction marks on the windscreen associated with standalone units. Now, the toughie - engine choice. Diesel beats all - even the bloke in the street will tell you this - it's faster and more economical than any petrol engine currently available - so a diesel it is.
This type of person will ask you, with a concerned look on their face, if your personal finances are ok if you're running a similar, petrol car. There will be some sucking through teeth when you mention that you like your petrol car, then a nervous, dismissive laugh, as one would give during chats with the residents at a local mental hospital.
Finally, everyone enjoys sunshine, so better get the convertible. Sign on the dotted line - it'll be delivered next month.