I've always used genuine BMW parts, may be the reason my repair costs may be higher than some. My experience has been that the quality is high and they fit as they are supposed to, and they support their warranty.
I agree that with so many vehicles made worldwide the typical parts should be available for quite some time. The real concern is those few critical parts, very unique, that if they fail there is a significant functional loss of the vehicle- such as DME, other computer parts or electronics. I don't have much experience with BMW Classic covering the "whole" catalogue of parts for a model, but if they do that would decrease my anxiety and allow me to keep it longer.

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'03 4.4 M62
Original Owner 107k on the clock
Alpine White/Beige Leather

Others:
2013 M-B E350 S4 Touring
2016 M-B E350 S4 Touring
2004 Porsche 911 C4S Track Duty