One of the reasons that analogue TV needs to be switched off is that the digital transmissions share the same bandwidth. Consequently, digital power can be increased when there are no analogue stations to interfere with - giving a better signal, and more of the spectrum is made available for multiplexes - so more channels.
IIRC, DAB is broadcast well outside of the current 88-108 FM spectrum - so that same argument doesn't hold true. There may be commercial / licencing / political issues behind a move to switch off analogue, but I don't believe there are any engineering ones.
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