It's certainly true that Euro6 diesels are a great deal cleaner than earlier iterations, but it's worth remembering that the economies of diesel only really kick in when moderate to high daily mileages are being driven, and for the vast majority of car drivers, most of whom live in urban and suburban areas, this simply isn't the case. For shorter daily mileages petrol is a much better alternative.
Then we come to BEV and hybrid technology, which has moved the goalposts. Diesels aren't competing against petrol any more so much as electricity, and it's difficult to justify the use of diesel where much more appropriate solutions exist, especially in the urban/short journey environment where most cars spend most of their time.
Times change, along with technology, environmental understanding, and politics/social priorities. What made sense 20 or 30 years ago isn't necessarily still the best solution today. We are starting to see some pretty major improvements in air quality in inner-city areas like London - 27% reduction in NO2, 30% particulates. This is bringing genuine benefits to people's lives.
