While I'd agree that DAB hasn't taken off in the way that many would like it to have I'd still argue it's pretty popular. 20+ million sets is a significant number in a country of 60ish million people, and yes, I'm aware how that compares to the number of analogue sets.
Adding a DAB+ mux is a lovely idea to encourage take-up, but what broadcaster in their right mind would accept a (perhaps) 50% discount on their transmission bill for the reduced number of bits required, in return for reaching less than 15% of the audience they'd get on a DAB service?
Comparisons to Freeview aren't really like-for-like, since Freeview is a controlled service with mandated hardware standards so they have a stick (as well as the carrot of the the HD mux) to convince hardware manufacturers to introduce DVB-T2 support. DAB has no stick. Freeview also has a thriving market for cheap, functional STBs, so your telly's built-in receiver no longer supporting the latest standard just means you stick a new external box on. Not an option with a great many DAB receivers, which don't have any external inputs and will therefore become bin-fodder if you somehow manage to convince broadcasters to take up your DAB+ mux. A goodly number of these aren't 20 quid cheapies, but 100+ quid shiny things they were convinced to buy.