I like to think of it this way. Any camera you get has an initial $250 drop in value when you open the box because now it's used. After that, these things only loose between an additional $250-500 over the next 3 years. You could resell in 3 years for over $2700 unless Canon decides they over priced this model.
If you got a 3 year old mark II 6 months ago at $2300 (which would have been a good price), you loose the same $250 once you open the box because its used, and in 3 years your camera will loose alot of value. A 6 year old Mark II will be worth less than $1000 most likely contributing to over $1000 in lost value.
If you can in any way afford the initial investment (which you can get back any time you want to sell your body and get out of photography), you will loose less money purchasing a new body every 3-4 years when a new model comes out than purchasing a 3-4 year old model at a discount and upgrading that with another 3-4 year old model 3 years later.