But should it not be the case that the people you refer to (elderly, pregnant, and so on) should be able to have a seat in standard class by asking SC passengers to move? These stickers are widely visible on trains, and I don't see why SC passengers shouldn't simply be making way for those who are entitled to a seat. Indeed, they tend to have plenty of extra legroom so are already improvements on the SC product.
Furthermore, you suggest that these passengers would move back to SC when asked to do so. Whilst I agree with this in principle, the notion of asking an 85 year old lady with a walking stick to clamber back several carriages because a seat has become available elsewhere on the train (particularly on longer trains like EC and Virgin) is completely unworkable. You're asking people to move around more when, by definition, you're offering them a seat because their mobility is impaired!
I think it'd be a sad reflection on society if those of us who are more able were not standing up to allow those who need it more to get a seat.