Being somewhat old and decrepit I get the whole nostalgia thing and one way or another it's cost me a few quid over the years, and if/when Camelot finally smile on me there could be a considerable shopping list to be serviced. I also predate Sierras by quite a margin, so well remember their introduction and subsequent competition work. That said, I simply don't get £600k for a Ford Sierra, not even with such a magnificent provenance. Will it disappear into the hallowed halls of a private collection never to be seen again, or will it reappear in a couple of years from now with a £750k price tag? If it goes into a museum, folk won't want to see it because of what it represents in Ford development history, they'll want to see it because it's the one that made £600k.
I'm not sure that it's a 'paradigm shift in tastes' so much as folk riding the crest of a wave that will ultimately collapse; it's often said about 'cheap' cars that 'if it's too good to be true it probably is', and I think that can go the other way in that these heady values surely can't be sustainable.