Talking with people who live in Delhi (in both English and Hindi), I got the sense that 'dilli' is becoming increasingly rare.

I am overall rather tired of the city-renaming craze. It seems like expensive bike-shedding at best. You end up with ridiculous scenarios like renaming 'Madras' to 'Chennai', where 'Madras' in fact is a native, historic Tamil name for the city, and 'Chennai' is the name of Telugu collaborator with the British.