Does that make any practical difference today?
I think the real thing that obviates this discussion is that 'acceptable' (noise, resolution, etc.) is totally subjective. Some people are perfectly happy with ISO 800 on a digicam.... whereas others (me, for example) have 24x36" prints on the walls, and want them sharp and detailed with very low noise.
You're right, "acceptable" is subjective.
I am not interested in resolution, either way. (Maybe if I had the possibility to use 30 MP, I would start cropping.) I am the kind of guy who shoots (processes and keeps) thousands of photos taken at ISO 3200 (from events). I am the guy who wants 3 stops less noise in his photos, but I have learned that the main cause of the technically bad look of a photo is not noise, it's light, not its amount, but its physical properties, like its spectral power distribution ((lack of) similarity with D65). Actually, I want to be able to shoot at ISO 12800, so that I can use my 70-200 F4 IS at 1 / 200 s.
If you are someone who thinks that noise is what makes your photo look bad from a technical point of view, you should experiment with your camera in broad daylight at ISO 3200 or higher to see what your camera is capable of. Then repeat with indoor lighting, like incandescent, fluorescent or sodium lights, or with dusk light. You should really ask yourself if its the fault of the sensor's noise or if it's the light itself that's just "wrong".
You can see here
photos taken in normal room light at ISO 3200 (no flash, but a fluorescent tube, some sunlight and lots of luck for catching the right moment when the tube was in its best phase). Or here
is one taken in daylight at the entrance of a hangar (this had lots of light, but still ISO 3200).
Does anyone believe that a lower resolution (how low? 2 MP?) would make the photos look better from a technical point of view? It would not. The problem with low light and high ISO is not the noise, is that the sensor can no longer give you dynamic and tonal range. It just can't do it because it has no light to "slice"; in other words, the tonal range quanta is an absolute amount of light, not 1/2^12 of whatever light is available. You can have whatever resolution and noise level you want, if you don't have lucky light, dynamic range, and most importantly tonal range
, you just can't get a technically good photo.