I don't think Leica has much competition for the M, it seems to exist in its own niche. Even the Leica S with all it's potential issues and lackluster customer care still sells, and the reason is simple: as bcooter said, it's the look of the files and feel of the camera that is worth it all.
I agree that such long wait times for simple parts replacement are unjustified for such an expensive piece of gear, but Leica is also not the only camera company struggling in this. With the exception of a couple, the whole camera industry keeps living in the past when it comes to certain (or all) aspects of the business.
The cameras will keep selling for now, because they still have unique and appealing features that people are willing to pay and compromise for. Despite this, I do believe that things need to change sooner, because with easy access to quality sensors and so many lens makers ready to produce stellar optics for anyone willing to pay, it only takes a bold entrepreneur with enough money and a vision to kill companies like Nikon overnight.
