Yes, the rubber will deteriorate over a long time.
It helps if you DO NOT try to keep the diff oil level completely up to the edge of the check hole. About 1/4" below the edge of the check hole is plenty sufficient.
Due to the close proximity of the ring gear to the hole, there is actually quite a bit of "wake turbulence" of oil thrashing against the plug.
You should see what happens if the plug comes out while the truck is in use.