Is it common that the teeth are wasted/used up after several decades or are there other typical causes?

That is so rare I've never seen it before. My guess is that the clutch splines have never been greased since the bike was new. How old is the bike and how many miles has it done? What does the gearbox input shaft look like?

Bike is 1984, very likely they have never been greased and should be around 80.000 miles. The gearbox input shaft is in perfect condition.
Strange in any case. In one Youtube they talked about a similar case with no root cause ...

I've seen this many times. 
More often on airheads, but sometimes on K, specially K11 with more torque.
And sometimes on clutches with a brand new disc recently fitted! 
With or without grease applied.
I've been told that there was a batch of defect discs with a wrong treatment on the teeth...possible. The BMW technical expert told me that on bikes with high mileage, the input shaft should be replaced by a new one if it had worn marks, because a new disc would not be a perfect fit on old teeth.
But I suppose that the main reason is the fatigue of the part, and the kindess of the pilot!

Didir18 wrote:Strange in any case. In one Youtube they talked about a similar case with no root cause ...

Somebody should cross reference the dates of these failures with the dates of crop circles appearing in the vicinity to see if there is any correlation.

I have seen that quite a few times, one bike in particular was in the habit of doing it about every two years with no known cause.

I've moved this thread from Troubleshooting - General to the more specific Troubleshooting - Engine & Transmission. 

Nothing to see here, move along, these are not the droids you are looking for. 

I had them strip on a K75RT many moons ago. When mine went they also trashed the transmission input splines.

Hoping this never happens to me - but am now curious how this issue is resolved...  Are new replacement parts still available (if so, where)? Or, are used parts the only option? Also curious about cost of repair (ballpark idea).

Yep - new friction plates are available. No idea what the cost is in the US, but over here they're just short of £100.

dbier wrote:Hoping this never happens to me - but am now curious how this issue is resolved...  Are new replacement parts still available (if so, where)? Or, are used parts the only option? Also curious about cost of repair (ballpark idea).

I strongly recommand to change the whole clutch, friction plates and covers. 
It's not an easy job to change it as it requires to remove the whole transmission. Do you want to do it one or two times?
Most, or all of the clutch covers are worn also when the disc is worn. It's more subtle but they are not plane anymore and are conical.
See the space under the ruler?




All the clutches I have inspected were showing this kind of wear: the one on these photo had only 20000km! It's a mechanical nonsens to put only a new disc on this.

So, the disc alone is about 100$, the whole Sachs clutch kit about 350$. Consider 8 to 10 hours of work depending of versions and options.
I also recommend to use an oilproof disc made by EBR, a bit more expensive but who can cope with the oil leak taht will occur one day, so you can delay the repair as long as you tolerate the oil dripping under the bike.
And I always check at the same time the flywheel gasket for leaks, that implies an extra cost to change the o ring and the nut on the shaft. Most of the time, the gasket is out of order.