Your surprise is based on the assumption that the F5, as a member of a once-dominant but now niche class of product, should be steadily dropping in value. If you back up and track the changing values since the inception of the model, this might generally be true, but the decline is by no means linear and predictable, and nor should it be really. There's going to be periods of price stability, and even spikes of increase here and there. There's also certain limits to how far prices will fall so long as Nikon still exists and still makes F-mount lenses.
Even if total demand for film cameras shrinks, so long as there is a market for film and film cameras, I suspect demand for models like the F5 isn't going to erode at a linear pace because the factors contributing to an F5's desirability don't really change over time, nor do the tastes of those who would be interested in the first place.