I think rangefinder 35mm cameras are a pain to use. I'm not alone, rangefinder 35mm cameras were never big sellers after the 1950s. The F6 is light years ahead. More lenses available too. Maybe just collectors and poseurs are buying the Leicas?
I've used Nikon SLRs extensively. I've used a Barnak Leica extensively, they are simply different cameras.
SLRs excel when you want to use very long lenses or need the benefit of motor drive.
An RF viewfinder never blacks out. That makes it perfect for in the moment shooting to capture Henri Cartier-Bresson's "decisive moment".
RF cameras and lenses are substantially lighter and smaller than SLRs. Not only does that make them easier to carry, it also makes them far less intrusive and immediately visible to a subject. (Try putting any SLR in your pocket and going skiing. You better have really big pockets.)
You don't buy Leica (or Hasselblad) first for their camera bodies (says the guy who just bought an M2). You buy them for their lenses. The Leica optics have a look that belongs just to them, particularly at wide apertures.
RFs are generally very simple designs whereas SLRs are much more complex. So, SLRs, by the nature of their design, require more maintenance. My Leica IIIf is almost 70 years old and has had two maintenance cycles I am aware of. My Nikons (I have 7 Nikon film bodies) require shutter adjustment, foam replacement, and regular CLAs every decade or so.
But, SLRs give you an almost perfect view of the final captured image dimensions without parallax.
SLRs are noisy. Leica RFs, at least, are whisper quiet.
One is hammer, the other is a surgeon's scalpel. Both have a place. That's why I own and use both when I do shoot 35mm.