For years the mantra has been "there is no spare rolling stock" And now that spare stock IS available there is a rush to scrap it so that we may return to the traditional "there is no spare rolling stock"
Providing enough capacity will always cost money.
I have no doubt that an HST▸ costs more to run than a 4 car voyager, but the HST provides more capacity and should perhaps be compared not to a single 4 car voyager but to a PAIR of 4 car voyagers.
The issue is also the cost of running HSTs compared to Voyagers. I believe this is why XC▸ do no longer make full use of the HSTs they do have (I remember back in 2009 there were 4 diagrams on Mondays to Fridays but now there are only 2/3). The conversion to power operated doors has of course taken one HST at a time out of service recently but XC reduced the Monday-Friday diagrams quite a few years before that started.
I think there might also be a looming skills/knowledge crisis as lots of maintenance engineers brought up with the HSTs retire early or take redundancy. Laira has certainly lost a few over the past year.
A 4 car voyager has 190 seats. An HST has 542 (varies on configuration - that is a Cross Country figure). So, yes, the comparison is to a pair, and stepping up to a train with twice the capacity is a massive step. Let's say 300 travel on a Voyager at 07:53 from Taunton to Bristol each day at the moment. Not uncommon. Nasty crowded train. Trolley can't get through. Change to an HST and it's much more comfortable - but it also costs the operator (say) 1.5 times the running cost. Will the passenger numbers increase to 450? And how about the train between Leeds (12:00) and Newcastle (13:30) - will numbers on that grow 50% too, especially when it's competing with new TransPennine Expresses which have just had a huge investment? Even the numbers do grow 1.5, are they "profitable" anyway - or are you just multiplying the number that HMG pays
DB» by that 1.5?
I've talked pure finance there ... but what about the wider economic effect, climate issues, etc.? I doubt they really come into the sums? What about government popularity and promises? Well - the important things there seem to be travel within and across the marginal areas we say swing this week just gone, and journey time, frequency and comfort to London. Northern Powerhouse has a strong rail voice ... does Western Gateway? What does the mayor of Bristol say and do about public transport? What about the mayor of Manchester?
I suspect the skills shortage for engineers is spurious. With a decent size fleet (and there's nearly a dozen castles which I suspect are not hugely different trains

) there's enough quantity to have a decent bank of engineers, with more trained if more are needed. We are a long way from the heritage line nightmare of a fleet of 12 locomotives needing 12 different spare sets and 12 sets of specialists.
With a production line already running to convert trains, yes, more to convert but those skills learned, and it's probably not unreasonable to have something of an attrition rate to get rid of "worst cases". I am not saying "don't scrap even one" - just I have the uneasy feeling of short term franchise and treasury single term balance sheets taking almost exclusive priority over quality of travel, sustainability, wider area economy, long term direction and costs, etc ... and so much more so in the case of Cross Country that's a franchise its owners want rid of, and doesn't serve the seat of government nor is it the major operator in marginal areas.
A further thought. Looks like I may be in Coatbridge again from 13th January. Flight up on 12th - £23.99. Train up £94 (or £189 for a faster journey with changes). Headline prices - factor in railcard, cost of getting to and parking at the airports, journey times, comfort, etc and the balance perhaps changes - but perhaps with extra HST capacity the railways could start to regain and fill that extra space with good, long distance business. Should be greener than flying, especially when taking to / from airports into account. And travel time is no longer wasted time with the ability to work, sleep, relax ... that's if you're not jammed against a bulkhead or next to a fellow passenger who's first class size but jammed into a standard class seat.