I must be in a parallel universe. I thought that the purpose was to convey passengers quickly and reliably. I'm sure that bus drivers would like a larger space, but they get one which brings everything they need within reach. Unless there are safely issues with the cabs (a matter for the ORR), I see no reason to remove them from future builds, let alone existin ones. They allow passengers to move to find space, OBS, whether buffet,guard or RPI to cover the whole train providing security as well. They also permit train lengths to be adjusted.
It is however true that operators seem reluctant to split trains at off-peak times. Following other threads it seems that modern units, essentially computers with attached hardware, don't take kindly to splitting in service. It confuses them.
If you want something more radical, why not a new build of 8-coach fixed sets for the Edinburgh Glasgow Intercity service with the existing sets being cascaded to accommodate growth in the Stirling routes and so on.