Point in bold have already been debunked in this thread: rewatch the picture I posted of the Pentax MX. I'm pretty sure the Olympus OM is also smaller than Leica Ms that unlike what they say are NOT small cameras, or light. Screw mount Leicas were smaller, not the Ms...I don't know why you keep repeating this despite of the evidence of the contrary. Regarding the weight, a M3 with a Summicron 50mm and a Mr4 is almost 1 kg, more or less like a K1000 with standard lens (not a MX or a LX).
The design isn't simple, in particular the rangefinder is a sophisticated device with many movable parts that can get misaligned relatively easily and if it's so you won't find it out until you develop the film.
This is the rangefinder of a M3 and a M5:
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For low light applications I do NOT recommend a rangefinder because screwmount Leicas, Soviet copies and Canons had a dime patch, M have the bright one that sometimes "disappears" in low lightl...I miss more pics with RF than SLRs.
Get an LX and you'll realise how outdated were the Nikon Fs in comparison to Pentax cameras.