Putting taste/preference/opinion aside, I wonder whether I'm missing something, either on the technical side or the artistic side.
My question only addresses the context of digital cameras with "modern" sensors that have high resolutions and high ISO capability (e.g., the Nikon Z6; 24MP and ISO going up to 51200). It also relates to non-professional use by a hobbyist with somewhat advanced skills.
The argument I'm considering is: why would I need/want to buy an 85mm prime if I can get "exactly" the same with a 50mm with some cropping? The high resolution of the camera and the reportedly high optical quality (in particular, sharpness) of these new Z lenses — and in general high-quality prime lenses — means that cropping should not be an issue. I think there is no difference in terms of light — the focal ratio determines the "intensity" of light arriving at each pixel (relative to the amount of light in the scene, of course); so, all else being equal in both the scene and the settings, the pixels in the cropped image from the 50mm will be equally lit as the pixels in the full 85mm image (only difference will be that there are more pixels in the 85mm image). But even if it wasn't the same, these modern sensors with high ISO capability make the issue less relevant.
I understand that what I'm describing amounts to favoring digital magnification over optical magnification (which of course I understand is not right). But that would be a more relevant factor if we were talking of a wider difference — say, 50mm vs. 200mm or 20~24mm vs. 85~135mm. A digital magnification of 170%, with these super-sharp lenses should be nearly-indistinguishable from optical magnification (maybe? at least for most practical purposes?)
So, again, my question is: am I missing something, either on the technical or artistic side?
As an additional thought: I notice that bokeh (or other optical features) may constitute a relevant difference. However, I'm not really sure: at the same level of quality of the two lenses (assuming roughly the same quality of materials and construction, and same level of engineering skill and effort in the design of both lenses), I guess the larger focal distance will produce a higher-quality bokeh. On the other hand, the deficiencies in the bokeh are typically away from the center of the frame, no? So, when cropping the central area of the 50mm shot, we're removing the parts where the bokeh was lower-quality, making the two setups equivalent?