I have had two Reds and a Disco suffer this failure after many years of service. All three had brush deck issues as well. Just replacing U11may not be enough. In my case all three brush decks needed repair including motor replacement on two of them. All three needed cleaning and gear replacements. The Discovery was retired and is used as a spare parts source because of other issues, the other two robots are still in use thanks to a more capable U11 replacement transistor and vic7767's bearing kits.
One of my reds is used weekly and the other is run once a month. Both are now 6+ years old and still running after repair.
I think the Discovery and similar (Reds/sage/dirt dog) are some of the best Irobot products ever. I will keep mine running before updating. My guess is the 900 series will be my next purchase.
Anyway, if replacing transistors and maybe gears is in your comfort zone your robot is worth fixing. If not try to find a cheap 500 series replacement. And eventually add bearings to the CHM.