The OP raises an interesting question: are petrols more future-proof than diesels? My hunch is that we'll see increasing VED on diesels because the politicians won't be able to resist the opportunity to do so. They will claim that it's to protect air quality.
Also, local authorities (especially car hating ones - I'm looking at you, Southampton) might impose entry charges for diesels. Buses and taxis will be exempt of course because public transport is good, apparently.
Generally, if there's a policy to switch from diesels, the only way it'll work is if they make diesels more expensive to run than is currently the case. Just my musings, you understand.
Bubba.