As someone who is also very familiar with this machine having owned one since it was released, I disagree with most of what you have said above. I respect your opinion but just wanted to add some balance in case anyone comes along and sees what you wrote and thinks that it's undisputed fact. It's not, it's just one person's opinion. The same can be said of the following.
So, firstly I'd be interested to learn how long you kept your machine before you sold it? I'd also be interested to know if it was a base level machine or if you went for the upgraded CPU or GPU? Mine is the 4.0GHZ CPU with upgraded GPU, 1TB SSD and 24GB RAM and it is an absolute beast!
The iMac has always been, and continues to be a desktop machine for "everyone". It's the core consumer product line of machine as opposed to the entry level Mac Mini and the professional level Mac Pro. That said, the iMac is so powerful these days that a great many professionals are also using it instead of the Mac Pro. It is most definitely not only intended for photo editors and movie buffs.
Additionally, this machine is a very capable gaming machine and I use it regularly for X-Plane 10 and Elite: Dangerous and play these at 1440p for the most part. I play some other games as well and although I would be the first to say I'm not a hard core gamer, this machine is without doubt the most powerful games machine I have ever owned and handles anything I throw at it, in resolutions well above HD.
Apple didn't make a big mistake bringing out a 5K display. If you understand the concept of hi dpi (or retina) displays, you will know that Apple needed a display with twice the horizontal and twice the vertical resolution of their previous 27" thunderbolt display in order to make the jump to retina resolution. So, 5120x2880 instead of 2560x1440. Also, if you want to edit 4k video then having a 5k display is very handy as it lets you have toolbars etc around the edges of the full 4k edit window. So this was no mistake but it was certainly a big shock to competitors who were utterly blown away by the fact Apple could sell a top quality 5K display at a price point of $1999 and throw in a free computer!
I don't understand why you say the 5K display looks "dog ugly" at 1080p because I certainly don't have that problem and I used to work for both IBM and another company who remanufactured LCD displays where I specialised in testing LCD display quality. Ideally try to run at 1440p as that is exactly half the native resolution but it works just fine at 1080p in my experience.
So, you're saying that an early 2009 Mac Pro 4,1 "destroys the 5K in performance"? Hmm, I find that a stretch to believe given that the 5K iMac can beat the latest Mac Pro in many of the benchmarks, never mind a 6 year old model. What CPU(s) do you have in your 4,1? Have a look on Geekbench
http://browser.primatelabs.com/mac-benchmarks The 4GHz iMac is top on single core and you'd need to go a long way down that list to find the 2009 Mac Pro. Even on multi-core benchmarks the iMac holds its own pretty well and is only beaten by the 8 core and above Mac Pros. Given that very few applications can make use of more than 2-4 cores efficiently the single core performance is generally more important.
Anyway, I'm not trying to suggest that your Mac Pro is a bad machine, quite the opposite actually, it's a great machine and it sounds like you're as happy with it as I am with my 5K iMac. But just because you didn't gel with the iMac doesn't make it a bad machine either and many people would in fact find it to be the ideal machine for them to buy.