Erik, it's not as simple as that.
For one, I very rarely have any images that I leave as 3:2.
When I shoot portraits, 3:2 is always too damn long or too damn wide and I am cropping away pixels I paid for. This is an utter waster IMO.
So for portraiture, I rarely prefer using the smaller formats anymore.
For landscape work, I work quite often with 16:9 aspect, so the smaller formats actually work quite well (Unless I am shift stitching on the MFD).
As I mostly use the MF kit for portraiture, I can never consider the Leicas as a serious option.
But most importantly, it is not just about cropping it in post, the way one visualize a scene (Fit subject to crop, as you have mentioned) makes a difference in how I shoot and I have heard the same from other photographers too, especially those who were using 6x6 a lot in the past.
Edit: Another possible reason is that 4:3 and 4:5 are closer to the most common print formats compared to 3:2, which is why a lot of photographers preffered these formats in the past. But in the current day, when a lot of images end up being displayed on a widescreen than getting printed, 3:2 has an advantage as well.