In most of Europe (the UK is one exception I know of) diesel costs less at the pump than petrol. The cars themselves cost a little more, but the fuel usually is cheaper.
Excluding hybrids, the current generation of petrol engines (as they historically have done) have a lower fuel economy that diesel. A new VW Passat for example can be configured with a 1.5l petrol engine which achieves 53.3mpg (UK) or a 2.0l diesel engine that achieves 68.9mpg (UK) [0].
Diesel fuel itself has around 15% more energy per volume than petrol, and the properties of its combustion just make it more efficient than petrol, so I don't see how this will ever change, unless a new method of combustion is adopted. I don't really see that happening though, as hybrid and EV technology will likely take over from pure ICE vehicles.
[0] https://www.volkswagen.co.uk/app/configurator/vw-gb/en/passa...
diesel cost MORE than petrol already even in Europe and modern diesel engines consume as much as modern petrol engine with automatic transmission, in past diesel used to be more efficient, but this benefit was wiped out