The comment about only using the shell is interesting as that to my eyes would mean that the current interior is completely gutted and replaced with new but that's not the case on the prototype and i'm not sure it will be the case on any that go into production.
Well it is a prototype - and given the nature of the work they're doing to them, which is primarily around the traction package, it's not really surprising they haven't done anything with the interior. The interior is the easy bit.
No obviously I haven't travelled one as yet but many including myself don't see it as a major step up in quality from a Railbus and it also has some disadvantages in terms of Low Top Speed,
Well, let's look at some of the 'knowns' -
The D trains have proper bogies rather than being fixed axles mounted onto the frame - so the basic ride quality should be alot better. I did travel on them occasionally in London and don't recall the ride being poor.
The lower top speed is a bit of a red herring. Given these are likely to be deployed either where there are frequent stops (in which case acceleration is more important) or the line speed is already low (e.g. Marston Vale is only 60mph now) - it's not a problem. Their acceleration ought to be far better than a Pacer and probably better than a Sprinter - so any loss in top speed will probably be more than off-set by the improved acceleration. I've used this example before, so apologies, but try being at Kings Cross when both a 313 (top speed 75mph) and an HST leave together. The 313's long gone and is slowing for its stop at FP by the time the HST catches it.
and many customer's on the Northern Franchise are fed up of the Large Fleet of Railbuses frequently used on inappropriate length Journey's and much of the Sprinter fleet which hasn't had a proper decent overall in years, so the idea of trotting out 'D' trains to Northern is seen as more crap for the North.
So your issue is less one of D trains and more one of inappropriate rostering in terms of what services units are deployed on.
Let's look in the medium term (i.e. 5 years). The GWML is going to be electrified which should free up some Sprinters from the Bristol and Cardiff areas.
The MML electrification will be underway which will release the 222s, presumably to supplant 158s, 170s or 185s - we don't know yet, but the trickle down effect means the 158s etc will be cascaded to release 150s. With the railbuses at the bottom of the pile.
Throw in some inevitable 'fill in' electrification (quick win schemes) obvious ones being some of the suburban lines around Manchester and Birmingham and a few more units come free.