Problems, problems. Why DAB will fail.
The biggest problem - cars don't come with DAB, and DAB doesn't cope at all well with the inevitable interference in a moving vehicle travelling through dead spots and surrounded by moving interference sources. Since at least half radio listening is in a car (I guess), they'd better not switch off FM.
DAB could, in fact, have been an improvement for car radio. What they should have done, is transmitted the DAB signal with associated lower-bitrate versions of the same thing delayed, say, five and ten seconds. Car radios would buffer the high-quality data and when interference kills packets, splice in the low-fidelity version received 5 seconds later (by which time the car will have moved). The short degradation of quality would sound much like interference on FM does.
The second problem - DAB isn't green. DAB radios eat batteries at a frightening rate. They really shouldn't have made them to run on standard batteries at all, they should sell them with built-in Li-ion rechargeables like mobile phones or iPods. There are now a lot of people who think that you can't afford to run a DAB radio. Yes, I do know you can buy AA 2700mAh NiMH rechargeables and AA to C or AA to D adapters, but there are a lot of less clueful types out there, and it's still a pain to have to fiddle around moving batteries between radio and charger on a weekly basis. My old FM radio ran for about three months on one set of AAs. (Shame it ended up drowned in my bath - which incidentally, is why mains-powered is never an appropriate solution).
The third problem - DAB fidelity is terrible compared to FM. Maybe MP3-loving pop listeners don't notice, but DAB turns classical music from a sublime pleasure into a pain. The encoding generates non-harmonic distortion - tones that have no musical relationship to the actual notes. FM doesn't do this. Harmonic distortion - especially low-harmonic distortion - is far more tolerable.
The only reason I own a DAB radio at all is that I can listen to BBC World service in my bathroom without the cacaphonic noises one gets on Medium Wave. But my next household portable will be an internet +FM radio, I don't really care if DAB is part of the mix or not.
The final problem no-one seems to be thinking about, is emergency public service broadcasting. An FM transmitter is low-tech stuff, and not at all power-hungry. You could run an emergency service for days off a car battery. I doubt you can do that with DAB, and in any case, the DAB radios out there would have flat batteries within hours, even if they weren't mains-only.
I remember the 1987 "Hurricane". There could have been panic afterwards, with electricity and telephones both knocked out in parts of the country. But the BBC could still transmit FM, and people had battery radios to listen with, and so everyone knew what happened and that civilisation would soon get back on its feet. (Soon = 4 days, in some rural parts!)