I have worn out diesels, The motor will "Give up the Ghost" with a diesel, generally not the drive train.
Per Toot: "Yes, if the drive is through the rods as seems the case with most, unless in the tender, those poor tiny plain bearings would take a beating".
I have never seen a drive train for a model steamer that did not use the side rods to link the drivers together. They could all be geared together and the side rods only used for detail; however, this would seem unnecessary and more complicated. I have seen 4-4-0s with the motor in the tender; but, it had a drive shaft from the motor to a gear box on one of the driver sets under the boiler. So, in this instance this really is no different.
This system of driving the drivers seems to be standard and works O.K. for the most part. However, I had a Broadway Limited NYC Hudson where the drive rods would cris-cross on occasions and lock the drive system up, right now! I never understood how this happened; none the less, it made the loco worthless and was enough cause for Broadway to exchange it for a new one where I assume they had worked this problem out. I immediately sold this loco on Ebay, as I no longer trusted BLI. I hope those of you with BLI equipment have had good luck with them!
These steamer problems, along with other problems I have had, makes me think the current producers of steam products are thinking more about how detailed their locomotives are and less about the sound mechanical aspects the locomotives. These thoughts are the basis for starting this thread. I have old MDC/Roundhouse locomotives which are going strong and I don't know how you could make them any simpler.