An HST is an impressive sight when leaving the station. it is an iconic image in the publics mind of BR days, I would argue that in 20 years time the HST (in original livery) will be the main image in the public mind of the diesel era on our railways. In the North of Scotland they will probably still be in service.
A HST is an impressive sight when its leaving a station and looking to reach 125 mph as soon as possible. The departure from Darlington is an example of this. A departure from Highley is not quite the same.
If I asked my work colleagues (who are all university educated with excellent degrees) what a HST is the vast majority would answer either a Eurostar or a Bullet Train and a few might say a TGV. Most of them would have no idea what our HST is and only a few will have traveled on it. Most rail travelers are interested in where they are traveling to and not what they are traveling on.
This summer three of my colleagues took their families for a day out at a preserved railway. They mentioned over lunch that they had taken their children for a ride on a steam train. They had no idea whether the loco was a GWR Pannier tank or a Bullied Pacific, all they knew was it was a steam train and to them that was all that mattered. If they had traveled behind a diesel in their eyes it would just be a diesel regardless of whether it was a HST or a class 45.
Ask any preserved railway and they will tell you that steam sells. In my eyes diesels deserve their place on our preserved railways but your typical none enthusiast family is looking for a steam train, especially when they are paying £50 for a family ticket.