Why not just have the person send back the coins and reverse it manually. Then u don't have to change bitcoin
But what if the seller is dishonest? You sent BTC and then didn't receive what was expected.
With reversibility, seller is at risk, with non-reversibility, buyer is at risk.
Its not just dishonest sellers. People are always having their bank account details phished. The banking system protects them by making good any money stolen by the phisher. If the idea is that Bitcoin can replace fiat currency, a similiar reversible system will be needed.