I think that's true of a lot of auctions, not just MV collections. I can recall Land Rovers going at Aston Down in the '90s for more than I could have bought an equivalent vehicle from the local dealer's forecourt.
I really don't think that's the case; it assumes demand will always significantly exceed supply. There are strong reasons for prices to have gone up in recent years; but looking at the history of collectables, particularly classic vehicles, the way things seem to go is that prices shoot up due to factors such as availability, interest in a vehicle, other ways of investing etc., but then prices level out or even fall. The trick is judging the top of the market!