Digital radio IS sinking everywhere despite the herculean efforts of the digital boosters who are trying to rake in the dough on the deal of course. Unfortunately for them you can fool some of the people some of the time but not all of the people all of the time. Digital radio is junk.
No, it's not. The reason growth has slowed in the UK is because it's become fairly ubiquitous. The ones who enjoy it have radios and the ones who don't aren't going to go out and buy them "just because". It's found a role as a supplemental service to analog radio, but that doesn't mean it's sinking any more than SiriusXM is sinking because they sell practically zero radios that aren't in a new car carrying case. Over 30% of UK listeners are now DAB listeners.
It's true that DAB and DAB+ has had mixed results from country to country, but it is doing well where it's been widely implemented. The Czech Republic has over 50% penetration at this point. Denmark is noted as having over 30% of the national audience solely on DAB at this point, with a plan to shut analog transmissions in the next few years. They're also migrating to DAB+ next year. I think France is trailing DAB+ since they were late to the game and they probably won't stick with it unless it does well in surrounding countries. However, DAB reaches 90% of Germans and is widely available as standard equipment in German automobiles, offering something like 120 streams of programming. France's other neighbor, Spain, has limited DAB access (something like 20% of the population can hear it) but their service seems to not be going anywhere despite the government's dire fiscal situation. Hong Kong has citywide service, and Ireland's national service is over the 50% mark as well.
Tiny Malta is a big DAB+ center. Their multiplex covers 100% of the population since it's a small islands nation and 25% already have converted to DAB listening. Both The Netherlands and Norway are all in on DAB; Norway has a 2017 FM shutdown date that so far they seem on track to meet.
The list goes on and on, not even including countries still testing out DAB or DAB+. It shouldn't surprise anyone that uptake of DAB is slow like HD radio in this country when trials often just repeat existing FM analog stations and may come and go without warning. It's not until there's a permanent slate of multiplexed content that uptake rises, and who can blame them? No one in the US bought HD radio just to hear the same analog station in digital. No, it was the subchannels that drove the dozen sales of radios.
And even though it's not DAB, it's worth noting that South Korea has wholeheartedly embraced mobile digital radio and TV — over 60 million devices are in use there with DMB, most of them cell phones. Free digital TV and radio via cell phone is something we here can only dream of, since we can barely even get plain old analog FM in phones anymore.