the biggest wallet cost is labor getting to it
Sorry, but I had that feeling.
My brushes were hardly worn at 34K. I know you trust your shop, but it is still only another opinion at this stage until it comes off. Alternator brush design is very clever to achieve high reliability for their high rpms. First they are only taking the low currents to magnetize the rotor and second the first brush always runs on the center of the brush contact ring plate, with only a small radius to the second brush.
I would have gone with a stuck brush in its housing, a diode, or regulator fault first with your mileage. Maybe a very remote possibility of the rubber 'cush' drive coupling. If the brushes really are worn down (which I doubt), then I would also have concerns for the bearings. Make sure they can show you the alternator and brushes once they get it off the bike. These are BIG heavy duty alternators and not known for brush failures at those mileages. I would not expect to be replacing brushes until about 100K.
Now I know you are locked into a dealer doing the work, but you may want to think about a couple of other things whilst the rear end is exposed, since they do not add much more labor time:
1. Check starter brushes
2. Check cost of replacing the clutch slave.
3. Only 'maybe' check clutch disc wear and replace clutch O ring.
That is the difference between doing something yourself and having a dealer do it. Doing it yourself you can get the most from your time and labor to do more for not many extra wallet beans. Whereas dealers tend to be focussed on individual service operations and their charges for those.
There is nothing special about those Bosch alternators from a reconditioning point of view. I had a friend who bought a new Asian (Bosch 'compatible') car alternator for around $100 and found the 'frame' was a slightly different shape and would not fit his car. He went back with the new and broken oem. They took out all the new innards including bearings and swapped them over in less than an hour. You get the same kind of service from a shop servicing Bosch alternators. Buying an alternator from BMW would be my very last resort. In your case the money you save would pay for your teardown labor.
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