I don't get the issue with depreciation. In my mind, you set a price at which you say "now I'm willing to buy it, the camera or lens is worth this much to me" at that point you simply go out and buy it. Once you have done that, you have the pleasure of using it. I bought the pancake lens when it came out at 280 euro. Now it's only 200. You could say that I lost 80 euro on it, but it does not feel that way. I have been taking great pictures with it and love to have it on my camera. I love the lens at 300 euro, I love it at 200 and I would not have bought it, if it was 400. And the good news is that if it ever breaks down, I can now get a new one for only 200 euro now instead of 280. Every month you have it, is an extra month you can enjoy what you bought.
Things can go down or up in value, but at least you have the object you desired. If the value of it bothers you so much, put the money to buy it it on a separate bank account and pretend you have bought it and hey presto, it becomes worth more every year.