How long will Douglas fir planks last if left untreated outside, not directly exposed to weather, but some moisture occaisionally
Plywood is actually stronger inch per inch because of the cross grain used in the core. However it will delaminates with repeated moisture as you have found.
I have resided a couple of old homes 20’s&amp; 30’s era that had fur sub siding that on occasion did get wet and it was still good this was back in the 90’s so a 20-30 year life span as a sub siding is realistic for sub siding and being solid wood you don’t have to worry about delaminating.
The only thing I could think of off the top of my head is the fur planks may be a slightly different thickness but that will probably be true with modern plywood compared 60’s thickness but the siding over the top will probably hide the small difference.
You might consider a quick brush on  copper naphthalene treatment on the exterior to stop decay for longer.