Thought about cutting off at the rusted out base and inserting a smaller post.  Only issue with that is availability of posts in the right size, and the hardware the fence sections use to attach to the posts is set up for 2" square.

If 2" plastic was available, would maybe try that.  These posts are only about 10 years old and rusted out at the base.  Weird thing, some posts are not rusted at all, some only standing because the fence is holding them up.

Concrete is good, no cracking other than one gate foundation.

Thought about the Bosch type rotary hammer, just not sure how well it would work.  Figured the big hammer would certainly do what I need without drilling first, etc.  Good thing about the Bosch, would have other uses.  Can always cancel the Amazon order, not shipped yet.

The posts Lowes and Home Dept sell are supposed to be galvanized and then painted, maybe the stuff the original fence guys used was cheap uncoated crap?  Local hardware is my only reasonable cost source, will forget getting aluminum, and the dissimilar metal thing is a good tip.

I would've thought the Bosch stuff would've been more expensive, but was in a similar price range as the one that you ordered.

Galvanized and painted sounds like good protection.  Just need to make sure to patch any nicks in the finish.  Not sure if you live near the ocean, but when we do work at Pearl Harbor, everything rusts.  Galvanized and stainless just takes longer to rust.   

Have you looked into the synthetic wood posts?  They look almost like wood posts, but are basically plastic.  I've seen them used a lot for outdoor stuff like picnic tables and decks, but not as fencing though. 

After considering one of the Bosch rotary hammers rather than the pure demo hammer, had another thought. 

Lowes sells a version of the 2" metal posts with a square 4 hole flange.  Though about maybe just cutting off the rusted out posts at the bottom, leaving the cement, and using the rotary hammer to drill and install concrete anchor bolts.  Assuming the bolts do not corrode all to hell, replacing the posts would be a snap if needed in the future.  Not digging and disposing of all the concrete might be worth it.

Never seen the fake wood posts, but if were to try, wife would insist on painting them black..  Working well is not enough, must look good too, of course.