Is the main reason audio equipment depreciates so much is

Definitely #4, over-priced to begin with. I offer the following example(s) as proof.

Consider the following brands of audio equipment, all of which I have. What is interesting to note is that not only do the manufacturers of these items claim they are directly comparable to equivalent products made by other manufacturers that charge many times (as much as 10X in certain cases) as much, but numerous professional and average Joe audiophile reviews also agree that their value is simply outstanding. 

Not in any particular order:

Emotive
GoldenEar
Musical Fidelity

Agree with over-priced to begin it. Companies keep churning out new products. Claiming this new version is the one. And audio reviwers with an eye on advertisment fees for the magazines play along. How many reviews have you read. This is it, hold the presses the wheel has just been re-invented. Make our purchases and before you get them into our homes, before their even set up, they've drop in value at least, 30% sometimes 40+%.

A case in point; Me. I bought Snell Type B's in 1994. I spent a lot of time with them. TONS of time experimenting with them. My ears became trained to their sound. I know them as well as anyone. I've been smitten ever since with them. It's 20 years and hundreds of speakers later. How many people remember or regard them with so much fondness? Hence the pathetic resale value of older gear. BTW; what is the average age of an A-goner? Joe (59)