I have read that a lot of the old wiring has insulation made from latex. I have personally seen this stuff chemically deteriorate and melt while sitting in the case. My Hother motor did that.
The wiring in many of these old 40s and 50s machines is aluminum. AL is a good conductor of electricity but it expands and contracts due to heat when in use and it will corrode at the drop of the hat.
So replacing the old wiring is a good thing to do.
Joe