Diesel does produce less CO2 than petrol, the problem is that it produces a lot of other waste that is more harmful making them worse overall. The focus on CO2 only was a result of lobbying from (mainly German) car manufacturers as it was easier to produce diesel cars than to bother to do any real research to tackle the unsolvable problem of pollution caused by combustion engines.
I think diesel will still be around for a while yet but it’s days are numbered in the types of vehicle that don’t really benefit from a diesel engine. Not sure how a ban makes sense though, might as well just let the market dictate it and car sales already show it is happening.