I don't think it is the only way, they could offer a diesel scrappage scheme for example, or change the VED to be heavily biased against Diesel, or expand congestion charging for diesels etc. I also doubt the short term commitment is as strong, so even if that was the only way I am not convinced they would go ahead with it the way you have described.
In essence I think there will an endeavour to reduce diesel emissions but I don't think it will be as drastic/rapid as you do. We'll find out next month![]()
With Brexit EU emission targets become less relevant with every passing day...
In all seriousness there would be uproar if if they took retrospective action against diesel cars.
They could just put a levy on new diesel cars, kind of like the opposite of the electric vehicle grant. People vote with their wallets but getting people to scrap perfectly suitable vehicles is the least environmentally friendly policy they could possibly make.