Max is mostly right. Parts availability for old beamers is better than virtually anything on two wheels of similar era.
I've been doing rehab on old bikes for decades, and in that time the cost of old BMW parts (believe it or not) has actually gotten LOWER.
Some stuff is OOP, but alternatives exist (many of which are better). Poor people can still economically ride 30+ year old beamers as daily transportation and not worry. Its precisely why I started collecting airheads again after a 12 year lapse.
Try to match that with any other marque. Seriously; go ahead and try it.