The HST's are not 'ancient', sure they are 40 years old but could have another 20 years service life if its economically worth while to resolve the issues they do have, i.e the main ones being the doors and discharge toilets.
Not everyone who retires stays retired. It know at least half a dozen people who are still working in their 80s having already retired and returned to work.
Regarding the HST's being squeaky, they are no worse than many a more modern train, including the Voyergers.
Crash protection wise the MK 3 carriages are still up there with the best, if anything they were over engineered and stil have years of safe operation ahead of them if you take into account the assessment done for Chiltern railways which concluded (if I remember correctly) that they have a 20 year life ahead of them. One thing is certain,they won't splinter to bit if they crash, far from it. The power cars are also in a good condition and are as safe as any other locomotive.
The top speed of the HST's is a 125, faster than many trains on the network. Sure they are not the quickest off the mark, but that certain doesn't mean they 'trundle along slowly'
The HST are train of the 1970's. In my view thay were way ahead of their time, still being considered by many as a modern viable train 40 years later. Other classes,such as the Voyergers were hardly leading edge when introduced. This is one of the reasons that they are highly regarded, they have staying power, are are still viable years after lesser trains have been withdrawn. When the Voyergers hit 40 (or even 30) will the operators have any work for them or be looking to upgrade them...I doubt it, but ScotRail and to a lesser extent GWR are doing that with the HST's, and that's not just down to a shortage of available stock.
One thing I do agree with you is in relation to the Cross Country operations, they are not really suitable for that route, and never really were. But that doesn't mean that they don't have future, as ScotRail are about to prove.