As a side note I think BBC national are the only major services still using microwave links, most service have now migrated to ARQnet, Arqiva's fibre network.
BBC National FM Tx dist is now on BT MPLS (still in a NICAM wrapper !) on the same fibre network that feeds the DTT sites. Where an FM Tx is not co-sited with a TV one, then there's a microwave link. For instance North Hessary Tor is fed from Plympton by microwave (Plympton is the nearest fibre fed DTT site)
As far as Arqiva is concerned, it doesn't matter if they're PSB or not. Global is one of their biggest customers, so they would see the loss of any of their services to be a major issue that needs to be resolved rapidly. I'm sure the listeners would also be quick to complain. You also need to remember that a significant chuck of listenership is in the car, the fast majority of which still only have FM/AM. In the home as you say most people are on other platforms now and it probably won't be an issue.
FWIW My two lads (smack in the middle of Global's/EMAP's target demographic) use 4G bluetooth streaming to their car radios, for all those stations. So do all their other non DAB car-radio friends. Just saying
Global's target demographic is pretty large when taking into account all their brands.
I was referring to Heart/Capital/Kiss etc, but I take your point