Siding on a house is a short-lived component, good for replacement about every 30 years. The remainder of the structure is very well worth salvaging as it was most likely built out of longleaf pine, which is 20-30% stronger than yellow pine used today, not allowing for the frequent knots in modern lumber which widens the gap. And the dimensions of the original lumber are larger than current (full 2" thick studs versus 1 1/2" today, 6x6 or larger sills, etc.), widening the gap even further. Obviously there would be some decay and insect damage to the old lumber but probably less than one would think - longleaf pine is much more decay resistant than modern yellow pine.