I really hate that engineers, regulators, and businesspeople managed to work together to make something simple and reliable less reliable but more "modern".
If it's optimal, the quality of a digital link will be pretty well perfect up until that cutoff (but no link is truly optimal). DAB's design placed emphasis on more channels rather than longer range, so the point at which DAB cuts out will likely be before the point that FM has degraded to unintelligibly. In theory there can be a region where FM has degraded but DAB is still holding up, but in the name of more channels this region is probably small.
You'd think that this would be a good scenario for 'hierarchical modulation':
> Hierarchical modulation is particularly used to mitigate the cliff effect in digital television broadcast, particularly mobile TV, by providing a (lower quality) fallback signal in case of weak signals, allowing graceful degradation instead of complete signal loss. It has been widely proven and included in various standards, such as DVB-T, MediaFLO, UMB (Ultra Mobile Broadband, a new 3.5th generation mobile network standard developed by 3GPP2), and is under study for DVB-H.
* https://en.wikipedia.org/wiki/Hierarchical_modulation
Combine that with scalable audio quality: