Vertical siding, and any siding, for that matter, should be looked at in much the same way as other enclosure materials, the very same way you would look at the roofing. They are all maintenance items, and periodically in the life of a historic building, they will reach the end of their service life and require replacement.
As a for instance the barn I have recently been working on (30 years younger than yours) has three layers of siding on it, two layers of vertical (the second as shot as the first) and a layer of clapboards, which likewise has reached the end of it’s time, and will be, in the coming weeks, stripped and replaced.
The other sides of that same die,
All sides do not weather at the same rate, the sunniest side/s and the north tend to age faster, and all four sides might not require replacement.
Like Scott, I do not want to cast aspersions, but…
It might be that your contractor, at the same time that he sees real value to the life of the barn in the replacement of the siding, he also sees other upsides to him, beyond the potential money to be made in its replacement. Completely stripping the barn will expose all the problem areas, and make it both easier to point out the extent of damage, and also easier to effect any needed repair or replacement. Simply in every way, (from his perspective especially) a situation far easier to project costs on, than piecemeal removal of limited sections. Do you have need of some known sill replacement which would require such sectional removal?
If things are yet to kick in, take a good light out into the barns interior during a wind driven rain. It could tell you much.
Also like Scott, I’d like to thank you for understanding that you’re not so much the owner of a historic property as you are its temporary steward. It seems to be in good hands.
Hoping I answered as many questions as I planted.
Best with it
