This topic is very interesting to me and something I've thought much about in my short tenure with Nihonto. I think Darcy and Rivkin both make good points and are both correct, but coming at it from different angles. I cannot articulate it like Darcy, but here's what I've concluded, and am curious if you agree.
Nihonto is art, and therefore not a commodity and prices are what the market will bear. The top 5 highest prices paid for a Monet ranged from 50M USD to over 100M. Now they are all Monets, and without being too presumptive, all are very desirable. But even at those prices, there is a different pool or "class" of buyers for a 50M painting v. 100M. Is the Water Lilies in the so called "danger zone" because it sold for half of the most expensive? Diamonds, Rolexes, gold, silver, these are all commodities, with very set prices. Art is not a commodity.
SO the danger zone in my opinion is overpaying for whatever level you buy at. Chances are with 8 billion people on this planet there will be people and a market for the Juyo sword that barely passed in a weak session, as long as you didn't overpay. Is it the tired Kaneuji worth less than a healthy one, of course. But if my budget only allows for the tired one and that is all I can afford, then I will buy the tired blade. I don't have unlimited funds to build a museum collection so I want to enjoy what I can afford. And if I didn't overpay I can sell it to the next person that is also in a similar situation. There's a big spectrum of swords and collectors that keeps the collecting of nihonto moving along.