So now that all the streams are no longer crossing and are in alignment, question still remains how loss of heat results in motor replacement.....
The heating system is rather simple. Engine operation produces heat, heat transfers to coolant, coolant flows through radiator (heater core), air blown across fins is heated then expelled through ducts. That's the way it's been for the last 40+ years (probably longer).
If the engine wasn't reaching operating temp or taking too long, a possible cause may be an open stat. More importantly, I can't imagine any DTC codes NOT getting set for this condition.
My question to the OP, what mechanical symptoms did the car present? How was mpg? Codes?