Hmm... I think a bit of clarification is in order here. I do not think that diesel fuel is going away. I do think that as the number of VW diesel cars on the road decreases many service stations will begin questioning the value of having a diesel pump that is only lightly used. Some will remove it in favor of an additional gas pump. The end result will be that in 5-10 years it will be harder (not impossible!) to find diesel, especially in city centers and suburbs, and you will need to do a bit more planning to locate a service station that sells it. Of course truck stops will always carry it so that option will not change.
For the record, I think the Sprinter is a great van. It just happens to excel at a slightly different set of criteria then the ProMaster, so knowing which is a better fit for your needs is the best approach. It is not a slam-dunk like comparing a Mercedes E-Class against a Dodge Dart where the extra money gets you a superior vehicle in every respect.
