I'm not convinced the government really are all that keen on the idea of removing diesel from city centres. It might happen, especially in London, as a side effect of financially viable electrification projects, but if we're saying that in 50+ years time, electrification will remove diesel from the city, is it really considered a priority?
If they're happy to put it off until most of us have gone? And if they're happy for diesel trains to run through our cities for the next 50+ years, why not the 50 after that? They could place a very large, very expensive order of bi-modes to get the job done in a fraction of the time, at a fraction of the cost. But it won't happen, because improved city centre air quality is not the main aim and there isn't a blank cheque for it.