How on Earth could you consider that arch as even remotely in the same league as a 60's concrete box?
Dark, chunky, imposing. Brutal. Which one am I talking about? Both of them. But at least the 60's concrete box is one of the classic examples of that style, unlike the arch and it's style, where pretty much every bit of neoclassical architecture from that era in London that survives (and there's a lot) is significantly better than the arch was.
People's love of the Euston Arch is:
1) it not being there and absence making the heart grow fonder
2) it being railway history - the main reason why its demolition was opposed in the 60s.
If some wealthy individual wanting to preserve it bought it 60 years ago, took it apart and rebuilt it somewhere away from Euston, it wouldn't be missed at Euston, and people wouldn't be overly fussed to visit it.
St. Pancras surviving presumably is as bad as 60's Euston, then?
Not at all. Rather than being the ugliest example of it's type, St. Pancras is pretty much the prettiest example of both the Gothic Revival architecture and a train shed in London (you can perhaps debate the Palace of Westminster and Brunel's shed being better than the Midland Hotel and Barlow's shed).