The success of many of the minimuxes demostrates there's demand for another tier of muxes, and they would probably be sustainable in most conurbations.
Muxco have also shown that, presumably via lower operating costs, or perhaps smaller capex because the technology is more mature, new services can be developed (see n.yorks or n.wales & chester mux as examples), which wouldn't have got a look in on the Bauer local muxes, full as they are of their own automated decades services, and apparently with much higher carriage fees..
Therefore I think, whilst this is a bold claim, it's a justified one, and could open a new tier of broadcasting for communities of interest in big cities and urban areas, as well as reducing the domiance of the big radio groups.
From a pratical point of view, following the reorganisation of the mux frequencies and additional tx'es, just how many spare frequencies are there on DAB in a city like Manchester, or other big cities?
Now the L-band is dead, would it be technically possible to license another city wide mux on a frequency which didn't affect geographically neighbouring muxes?