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Surely EMT or WMR would be just as likely to stop services at Birmingham, and leave out Wales if there were any issues.
And TfW would be more likely to stop at Birmingham the other way if they had problems!
So split the service, TfW runs Cardiff to Birmingham and EMR Birmingham to Nottingham, where the respective operators have more of an interest in their part of the route. TfW already run other services into Birmingham, and it's not too much of a stretch for EMR to cover to Birmingham given they already run services to Crewe, Manchester and Liverpool. Although WMR might want Birmingham to Nottingham, I think EMR is a better fit.
Of course this is all subject to how GBR shakes down in the future with the end of the franchises, but there will probably still be regions, and splitting this service like this would still make sense to me.
Of course, that's not great for through journeys (which I actually use, so this wouldn't really be in my interests), but it does feel like two routes stitched together, especially with half the services just doing Nottingham to Birmingham anyhow. However, I expect it might cause problems with platform occupancy at New Street, although perhaps the services for the two legs could use the same platform with a dwell time of 5 minutes to allow easy both ways connections, save platform space, and keep their respective current pathing, or some such arrangement?