One reason I love Leica and Zeiss glass so much is the look those lenses give me. There is a different look you get from Zeiss and from Leica glass. Some prefer Zeiss and some prefer Leica and that can very from lens to lens. A lot of modern lenses especially the Japanese lack a lot of character. Modern lenses tend to be computer designed and mostly about charts and sharpness and less about the look the lens generates. Leica M lenses are designed by people and a long history (almost a century) of lens design and what gives lenses certain characteristics. The only modern Canon lens I really loved that I owned was the 200 2L. In my opinion the three jewels in the Canon stable are the 200 2L, 300 2.8L and 400 2.8L. Leica M is not a camera one buys to do the kind of work that demands the reach that those lenses provide. All 3 are great lenses and each has a little different look to them. Where I say Leica M excels is the normal (50mm) lenses and the wide range though there 90 f/2 APO is a real gem. And their lenses will give a much different look than say Canon.
What also could be going on with a lot of this comparing the 85L with a 56 is they are different lenses and have different characteristics. They provide a different look. I do see a difference in the background. The ff has less DoF and more background blur compared to the 56. And comparing 100 kb jpgs on a web site is also not the way to go.
This comparison is always a silly one. Again what is important is finding equipment that best suits the way one sees and works. I tend to natural see at a slightly wide FoV and in tones and shapes so a Leica MM and a 35mm F/L is all I need to do my personal work. That doesn't mean I can't see and work in color. Most of my pro work is in color and though about 60% of that I shoot with a 35 on FF I still do a lot of work with 90,75 and some with a 24. But I would have no problem shooting with a crop sensor if I could get what i needed from it. I shoot Leica M (2 M10s, M 262, M-E and MM because they don't have big menus, small and unobtrusive, (M 10s have ISO dial on the left top, aperture on the lens and shutter speed on the right top) and I do not have to turn off automation. Great DoF scales on the lenses. And what is most important is they fit for the way I see and work and just get out of the way and let me create.
I have shot manually for over 40 years and I can make much better decisions, faster than any computer, for the way I want my finished work to look. I shot with Canon DSLRs for over a decade (and I shot with F-1s, 500 C/Ms, Leica Ms, large format and a lot of other manual equipment in the film days for several decades) never warmed up to DSLRs. Way to much stuff for me. But thats me. Many need what that world can offer (DSLR world)because that best fits the way the see and work. So if a crop fits the way you see and work go out and use it and don't worry about silly things like FF dies have less DoF. It shouldn't change what you are doing because if a crop mirrorless camera is the tool for you the rest doesn't matter one bit. In this world it is really nice to have real choices. Enjoy it because it might not last.