Granted, you can do these things with an SLR
(Chicago, Art Institute, hip shot with M3)
(Denver, park near the library and Art Museum, with my M4-2)
(Café tables near Sixth St, in Denver, with my M4-2 again)
Sure, an SLR could handle these shots just as well. But then, there's the little aspect of conspicuousness, discretion and creativity. I was able to walk by all these places, take the photographs without any more fussing than making sure they were in focus, and keep going my way. Granted, I also had to think about the exposure, and either pre-meter or guess. Fortunately, the fac that C-41 film is so forgiving also helps. Not to mention that the little voice in my mind was reassuring me all the time with the famous mantra "This can be fixed with PS later."
In short, while SLRs allow the same (and more) technical possibilities as the rangefinder camera, the latter is better for your brains. They pose challenges that the SLRs never met: wider frames to compose in (compare a 135mm top image area to what an SLR can do... up to 600mm!), use of the brain, and ease (no need to do a lot of camera adjustments on the spot). Besides, they're a lot quieter!
It seems, then, that the best way to avoid Alzheimer's is to use a Leica.