Bitcoin had a hard fork in the past due to bug in core protocol (integer overflow allowed to create bitcoins out of thin air).
Is this a bug in core protocol, or just somebody badly programmed or secured a smart contract?

Where would you draw the line? $100K lost due to bad smart contract will not be hardforked, but $1M will?

Nope. Everybody should be responsible for their own mistakes and those could be costly sometimes. With FIAT one small programming mistake could also cost you or your company millions (like Knight capital lost $440M within few minutes due to "trading glitch"), so crypto is not different here. Deal with it. Real world does not have hardfork options to fix your mistakes