I was at the Oregon State Fair yesterday, and one vendor had a system where
you pour your concrete plug into the ground and have a square concrete post
coming up a few inches out of the ground. Fence post gets bolted to that.
Fence post never touches the ground. If fence post rots away, you unbolt it
and bolt a new one in it's place. I'd be interested in doing this if I could
find instructions and parts.

Just remember that the bottom of your fence post needs to have a lot of
bending strength to resist a wind storm. So if the connection at the
base will break at near the same loading level that the post will break
at, you might have something worthwhile.

Do you have a link to the hardware in question?