I think fixed prime lens cameras are quite much unbeatable in image quality if they have top-notch lens design combined with great sensor and lens-sensor matching is done meticulously. There are such cameras. Ricoh GR, Fuji X100S, and Sigma DPM series just to mention some. But much more is needed for a great camera than pure IQ. That is useability. I think Sigma DPM series have the IQ but they are lacking in other features and their images are difficult to post-process (I have had DP2M). But what is really fantastic with Sigma’s DPM cameras is that they have three variants of them. DP1M, DP2M, and DP3M. 28mm, 45mm, and 75mm equivalent lenses. I can easily say that if those Sigma’s would work better in high-ISO situations and if the post-processing of their files would be easier I would have all three of them at the moment. It is as simple as that. Unfortunately, Sigma has not nailed this camera series in those two above mentioned areas. It really is a pity.
I have now Ricoh GR which has a 18.3mm (28mm equivalent) lens and the GR has all the bells and whistles I need. The GR is so great to use. Now I would like to have another GR with say 50mm (75mm equivalent) lens. Just in a way Sigma’s DPMs have. That would be a winner combination. Then I would have two GRs which together still are smaller than many mirrorless interchangeable lens cameras with two prime lenses.
Two GR bodies with fixed sensor-matched primes. That’s my dream. I could even see that happen.