Agreed. Motors generally don't stop working and it's the brushes that literally wear out. Way back when while working at a manufacturer, we were always rebuilding the grinders. If it wasn't the brushes, it was the shaft right off the pinion that broke (as they're designed to do). The Dremels have both brushes and a sacrificial grove on the shaft to save itself. Dremel recommends changing BOTH brushes at 50-60 hours or at 1/8" left, which ever comes first.
If it's not the brush, I'd say it's the motor's overload...but I don't think the little Dremel motor has them. Either way, be careful trying to "hog" out material with a Dremel hand tool. They're just not cut out for heavy work (no pun intended). If it was a big window, a band saw blade handle and a straight edge (2x4) gets the big work done and you can finish off with a rotary tool.