These are all good points
I love the HSTs, and they are definitely a British icon. But I do (grudgingly) feel that their replacement is justified, not because of age, not because of their ability to perform their role (I find them amazing at what they do :P and I haven't found many people who disagree with me), but because of their resource (not just money, but fuel, replacement components and so on) cost to operate, which is becoming very expensive, and also because of the electrification of much of their routes which makes them less preferable in almost every operational and environmental factor.
I'll miss them a lot when they do go out of mainstream service

, but I feel it is time for them to be replaced, and they have done very well to get 40 years of frontline service :P .
However, what I feel would be great is if one or two were kept in service, running a dedicated, low demand service (such as a Paddington to Bristol service and the corresponding return on a Sunday afternoon) that changed every two months or so. This would mean that a few select trains would stay in service, running services which were still part of the national railway service, be fairly easy and cheap to operate as they wouldn't be put under too much strain, while bringing some 'heritage' to those services: in a way, similar to what Chiltern have done with their 121s, or what Virgin unintentionally did with the Pretendolino :P . The new stock would take over, but one or two HSTs would be kept and maintained to a good standard to 'keep the flag waving' for as long as they could be maintained

Pretty sure I've posted that somewhere before, but it's just a little thing I wish would happen :P