Yes, from my experience you are both right on the money. Once the slipring surface on the rotor gets too worn, it gets difficult to get the brushes to work out well.

I remember several years ago, benzmac posted that he would replace the VR the first time the brushes wore out, but could rarely successfully reuse the alternator after the second VR wore out. He also, if I recall correctly, said that he would replace the alternator if the brush groove was deeper than an 1/8".

Good luck,