Our 1965 DC-area home has original aluminum siding that is dented, scuffed, with drain-holes painted shut, and is so full of other holes and caulk-bodged window / door flashing, etc. that we are getting bids on having new vinyl siding put on. In the attic it's easy to see the original exterior sheathing, something like a 1/4" thick fiber board. I can also feel it inside a defunct vent fan duct on the outside of the house that is plugged with insulation but otherwise allows access to the wall cavity. Phone camera even managed to capture it:
I assumed the right move here, given the age of the house and the best-case condition of this stuff (brittle and crumbly) is replacing with 7/16" OSB and new house wrap. This process would also give us a chance to address any bays empty of insulation, clean out mouse nested insulation and replace, etc. And the OSB will take fasteners for the few spots where it's necessary to put them in. (We've got hide-a-lines for mini-splits that run up the walls, and a standing pipe for the radon mitigation fan, for instance.)
Is there any reason we would want to consider leaving that existing sheathing in place and just wrap it (other than trying to save the $3k=$4k extra cost that adds)?
To be clear, I am not trying to avoid that cost. Just wondering if there aren't other issues / consequences that I haven't thought through that would argue for leaving it if it's in OK condition for the most part. (The stuff I have seen is in surprisingly good condition. It's brittle and old, but not rotted. But I'm sure there are spots that are rotted.)