Personally I would never buy a 7 year old back. Casting aside the fact the technology is old, I would not want to invest several thousand on an old piece of equipment when I could buy a brand new dSLR for a similar or likely less cost with 90% (or more in some instances) of the performance, a full warranty and support going into the next 5-7 years, more exciting because it's new and also because it's brand new with no surprises or mystery from mishandling, drops, etc. I can simply exchange for a new one if things are wrong. The Phase One DF and DF+ while capable is again a limiting platform as it's a dead end. I personally don't like and never have liked that platform. The depreciation is terrible now, it will be even worse in another 5 years.
There is so much going on in the industry, in almost all brands and formats, and my old gear is "good enough" to wait and see how things develop. This is another contributing factor I believe and another reason why I have no need to buy something now, particularly when it is old gear. With all this talk about small formats heading into 100mp+ - 200MP then I think it would be best to wait and see how technology fairs in the next year or so rather than invest further into a 7 year old 40MP CCD system with an old discontinued platform.
Then there is the large transition to motion that is occurring. Spending thousands on a system without at least 4K capture isn't a savvy option right now, unless it's offering something over and above, like 80-100MP on a large sensor.
There is certain gear you can buy which either holds it's value well or actually increases in value. Otherwise, unfortunately we need to accept that any gear that has technology as it's feature (ie. 99% of all gear today) is going to be next to worthless in some amount of time. after shooting for 20 years I have several dead systems that I could sell for pennies if I could only find a buyer, all gear that was expensive at the time of purchase. If you can make it work on a financial level, you are far better to cycle your upgrades sooner as to regain the maximum possible. Otherwise, it's status quo , I'm sorry to say, but I do empathise with you given we are all in the same boat.