Everyone is talking about the age of the rear Deore mech or the XTR chainset from the 90’s etc. etc. that they are still using. Its irrelevant if you only ride the bike 2 hrs a week in dry conditions, it will last for ages.
M900 XTR was mega at the time but the new stuff is light years ahead in functionality, build quality is subjective as manufacturing processes have improved dramatically over the years however the latest XTR and Durarace Di2 is jewel like and works flawlessly (I have both). The old XT and XTR “V” brakes would rattle their balls off after about 8 hrs of riding and the XTR pedals from the last series used to fall to bits and the light road wheels would snap spokes regular and they are under so much tension the wheel would be like a pringle.
I have two MTB bikes, one for training and one for racing. I do about 10-12 Hrs a week and I go through XT + XTR stuff after about two years with zero mechanical sympathy in all conditions so I am more than happy with that.
PS. Shimano fit it and forget it.