"...putting a new brush/regulator assembly in the alternator. Problem solved, WHY!!??"

The old brushes were worn enough so that they just barely touched the slip rings. This meant that they were energizing the alternator just enough so it would output a trickle of current to the battery. The flickering you saw in the indicator bulb was because the charging current was dithering around a few amps -- not enough to fully charge the battery, but just enough to provide running current to the system.

Your battery would have expired soon, particularly driving at night in the cold with the wipers going (BTDT). And a million miles from civilization and cell towers.

You could have soldered replacement brushes into your original assembly, but replacing it all is a safe approach.

"...wife calls it a serious psychological flaw..."

Being perceptive and curious enough to catch a problem in the nick of time, before it catches you, is a "psychological flaw?"
--
Don Foster (near Cape Cod, MA)