Yes, jaomul kind of hits at what I was trying to show with the DxO Mark test results...since the 5D was current, crop-sensor cameras from Nikon, Sony, and Pentax have seen a major increase in performance, due to the new sensor-manufacturing process that Sony (and Toshiba) have spent hundreds of millions of Yen to implement. So, in many ways, smaller sensor Nikon, Sony, and Pentax cameras have equalled, or surpassed, the image quality standard the 5D set back in the mid-2000's.
The desirability of a mid-2000's era Canon FF body kind of depends on exactly how and what you want to shoot. One thing I find is that FF bodies make the way LENSES perform very different. The 50 goes back to being a "normal" lens; the 85mm becomes an indoor-capable medium telephoto; a 70-200 becomes MUCH more-useful indoors in smaller rooms and in homes and office situations; a 24mm once again becomes a wide-angle lens. Backgrounds are no longer so doggone in-focus and recognizable as they are when using a smaller sensor and a short lens. In cramped shooting areas, FF makes the muslin or seamless paper LESS-recognizable, less-intrusive, when shooting with studio flash at f/stops like f/5.6 to f/8.
APS-C and FX digital are actually two DIFFERENT formats. I think sometimes APS-C is better, and easier to work with. it gives more depth of field per picture angle, and many times that is a huge benefit. At other times, it's NOT a benefit. The view through the viewfinder on FX is also larger, and easier to see, and that can be a plus for many people. But, OTOH, some of the high-end amateur-oriented APS-C cameras have a really NICE, big, high resolution, 3.0- or 3.2-inch rear LCD and live view composing and live-view focusing and focus magnification, for people who shoot that way. The 5D classic has a small, low-resolution, basically CRAPPY rear LCD for review, and no live view, and no video either. No in-built flash, no commander mode, basically a stripped-down $389 EOS Elan body with digital guts.
The EOS 1D Mark II on the other hand is a HIGH-end camera body. It's older, yeah, but it's fast, solid, and for some uses, would still be quite usable. it's a FABULOUS camera...with an older sensor. Sometimes the "camera" part is more important than the "sensor" part.