Maybe Angel's human side is what's keeping the Hellmouth closed? If he leaves, the Hellmouth opens. A greater-stakes play on the meta commentary star-crossed lover scenario.
Essentially, keeping the Hellmouth closed requires a caretaker. And that caretaker has to be a rare breed of demon that has happens to have a soul (in this scenario half-breeds like Doyle would be quite rare). Making any movement to free Angel difficult.