Stuff get's old..
I think there is a point when many Leica M's won't be viable, simply because of dried lube, haze in the optics, and hi repair costs.
I have been looking at M4-2 and P's simply because they aren't 40+ years old like M3 and M and M2, and they are silly expensive as users for 25 and 30 year old cameras.
Japanese (at least Nikons in my experience, don't cloud up, and dont have unreliable shutter mechanisms-- there is the FM2 that can be cagey... but if it mis-behaves throw it out and buy another!).