From the passengers point of view, voyagers were one of biggest downgrades ever. A popular newspaper at the time stated that the then Virgin cross country "had been given a record subsidy to halve the length of the trains"
Prior to the voyager fiasco, a few doubters expressed doubts about half length trains without sufficient luggage space or a proper buffet, such doubts were drowned out by the "shorter DMUs▸ are wonderful" brigade. Though of course "shorter" is not a term that should be applied to new trains ! "flexible" sounds so much better.
It is now fairly widely accepted that voyagers are not as wonderful as was promised, but no one is going to scrap relatively new trains so we are stuck with the wretched things for years.
I think there are several things being mixed together here.
One is the "promise". I'm not sure there's much point in comparing a new product of any kind with the vacuous marketing pitch made for it at or before introduction. You could do the same with anything - shoes, ice cream, mortgages, holidays, governments - and get much the same result. It might tell you a lot about marketing (though only what you already know) but not much about the product. I'd ask what reasonable expectations were not delivered.
Then in this case there is the trains' lengths, but remember why those shorter trains were chosen. I can remember the explanation: that the service interval would be halved. In round numbers that was from 1 tph to 2 tph for the core, and 0.5 tph to 1 tph for most of the rest. I'm not sure how close to that the actual service change came, but a quick look at a 2000 timetable suggests it was broadly achieved.
That in turn was based on the observation that the majority of customers, however you measure that, were doing short not long journeys. Also, as a lot of them were connecting to or from other local service, a higher frequency was a lot more valuable to them than a longer train.
If you want the same number of trains but longer, that's a matter of total capacity versus cost, which is a different question and a different kind of question.
Finally, of course, there is the detailed design of the trains, which may indeed merit criticism. From my earlier remarks you could say they are long-distance trains but not, on average, full of long-distance passengers. But, as this thread (before it digressed somewhat) has shown, relying on averages can be a mistake. Efficiency, such as in the allocation of space in a train, conflicts (in general) with flexibility, such as coping with a wide range of mixes of passenger types.
