I think we can throw something else into the mix here to keep it competitive.
Well, since I'm looking for repeatable removal, like Karn is, how about one of these options?
When Demolition Golem dies, if it was dealt combat damage this turn, return it to the battlefield.
(When a card switches zones it forgets its history right, so you couldn't loop with a sac engine)
When a noncreature permanent is put into your graveyard from the battlefield, you may return Demolition Golem from your graveyard to your hand.
When Demolition Golem dies, put it into its owner's library second from the top.
Edit: When Demolition Golem enters the battlefield, choose one or both: Destroy target permanent with converted mana cost 3 or less; and/or destroy target permanent with converted mana cost 4 or more.
Might need to cost 7 if we do that though
I like 7 though. I feel I could use another 7.