I recently had to replace the side wheel motor assembly on a Roomba. The motor and housing were fine, but the gears inside had worn out. I think they were just made out of ABS, so if they were printed in Nylon they would be a lot longer wearing. Roomba do make their robots (at least the 620 I have) repairable and a replacement of the whole assembly was only £20 so I'm not sure if the margin is there, but it is a bit wasteful when the only issue was the gears.

I've also considered creating some upgrades to the stock parts, but it's on my "some day I'll get around to it" list.