There are 2 things that don't lose value over time.
1. Something that's already at the minimum value, to the point where it's almost equivalent to its high alch price.
2. Something that's no longer available, and not made available again, but is tradable.
But a high level / expensive boss drop? As supply increases, prices go down.
Simply put, if 1000 people want to buy an item, and there's only 10 of them, people would be willing to pay more to get it.
If 1000 people want to buy an item, and there are 1000 of that item floating around, with most of them only wanting 1, then most people will get it without having to offer a premium rate.
Even after those 1000 people get their items, the supply will continue to increase, with fewer people with the current means or desire to buy them.
As the price goes down, more people may consider purchasing it, which may stabilize the price for the moment, after which, the trend repeats itself.
Updates can influence the price of items positively or negatively, obviously depending on the items relation to the update.
Simple basic economics.