First and foremost Vieri,
Many thanks for this excellent article! I really do appreciate your knowledge of subject and different camera systems, especially the Leica SL system and I understand that this is (currently) the system of choice for you!
Having said that I think one needs to kind of go with time and see what younger systems/ideas can offer and bring to the party - even for professional use! I mean the advances with smaller than FF sensors - namely APSC and m43. I remember that even Dr. Karbe (the head of optical development at Leica) stated in an interview regarding the Leica T lenses that if Oscar Barnak would develop (invent) the Leica today it would most probably have APSC format, mainly because this allows for much smaller lenses if developed specifically for this format. And IMHO all the T lenses are stellar performers, only issue (for me) is that the range is not complete enough.
On the other side especially Fuji has succeeded in that respect - their X system is from the ground up developed based on an APSC sensor and thus all their lenses are optimised for the system and I at least would be really hard pressed telling the differences between a Leica lens and a Fuji lens (and this comes from a year long Leica user perspective - digital as well as analog, but also extensive use of the Fuji X-system, especially with the stellar X-T2 and the 100-400, a lens that is unfortunately missing in the complete Leica setup). The main differences to Leica - Fuji is much cheaper and has a much, Much, MUCH more complete lens lineup for the X- system. I was a year long Fuji shooter as well (in the moment and for now completely switched to Olympus with their Pro lenses - I know this is a system that you would not even by any means consider as a serious camera system just because of sensor size
), but the Fuji system that might come closer to your standards is nothing less than stellar - WRT to IQ, lens system, AF, video and several other reasons.
So IMHO this proves that a decent APSC based system like the Fuji X is close or even on par with a FF Leica SL system - at least for today. Will this change? I think so, just because FF has so many sensor size advantages that it will easily allow alway for better IQ compared to APSC - even in 10, 20 or 50 years from now. But the issue is what does one really need? Stating that a great system has to be FF shows me some sort of ignorance WRT where camera, sensor and lens technologies are evolving. My prediction is that in some years from now even m43 will have arrived at an overall level that is far above what every photographer and filmmaker ever needs (8k or more?).
So while you favour the FF Leica SL, I cannot help but say that the current Fuji APSC offerings (especially with the X-H1) are on par with that level or even exceed in some (many) areas. The biggest obstacles for the current Fuji X system are still the far too small batteries and some limitation of the AF system - but hey, the Leica SL AF system does not even come close and so does their video implementation not come close to Fuji (or even the mediocre Olympus EM1.2 or Panasonic GH5).
Having said that - enjoy shooting with whatever system you prefer and love, because this is the finally most important point - it makes you grab your camera and shoot!