Well, Nvidia has promised DX12 support for cards as far back as Fermi, hasn't actually happened though, and that promise is now over a year past due. I disagree that they are actively sabotaging older cards, just a cost/benefit for them, and going back that far in GPU terms is asking quite a lot.
AMD is sort of known for releasing drivers over long periods of time that gradually increase performance over time. Nvidia is known for getting almost the maximum output of their cards for popular titles immediately.
You can look at that in two ways. An AMD card might see improvement over time, but then again, where has that performance been since you owned it? With Nvidia, you get what you get when you pay and the prices are usually higher.
Both companies occasionally make driver mistakes, more recently with Nvidia and the whole 'removeable' device fiasco and failed updates (the first few versions of the 1000 series drivers failed to install/work with older cards, probably rushed it out the door)
I've only owned one discrete AMD GPU in recent times, and I didn't have many issues until I ran into a scaling issue. A suitable AMD replacement was likely available, but I do enjoy the lower power requirements from Nvidia.