Sacrificial Pact's absorption barrier, with enough HP scaling, can easily get up to 150000 HP of absorption for a poorly geared Warlock, and possibly 250000 or more for a well-geared one. Considering that a non-discipline priest's Power Word: Shield, with decent gear, gives around 20000 HP absorption, this is an incredible amount of damage absorption. Though it only lasts for 10 secs, it has a 1 min CD.
Compare this to Dark Bargain, which grants 10 secs of immunity, and have 50% of the damage dealt over another 10 seconds with a 3 min CD. 250000 HP absorption from Sacrificial Pact, unless targeted by 3 players, will easily last the 10 secs that it is given against any 1 or 2 players. Undispellable too. In short, it serves the same function as Dark Bargain in a sense, with no after-damage DoT, or a CD 3 times as long.
Now let's see Soul Link. For a 250000 HP Warlock, that would grant around 62500 HP only, and it also reduces our self-healing by dispersing it with our pets.
There is just no reason to pick anything else over Sacrificial Pact unless Sacrificial Pact is nerfed (it already got nerfed from 300 -> 200%), or if Dark Bargain and Soul Link both get buffed.
Make Dark Bargain's CD be reduced to 2 mins, and extend the DoT to twice as long, and maybe it's worth getting.
Make Soul Link no longer split healing with pet, and increase its damage mitigation by another 5%, and perhaps it is worth taking.
But to be honest, Soul Link really, really deserves to a baseline ability. Even with all the regen we get, our damage mitigation is horrible compared to Shadow Priests and Frost and Arcane Mages.