The Inside Jab

[gleaming nest]

This might be a straight upgrade in most situations because how often are you killing more than 1 very important target before dying or leaving?

Spy can choose their targets, no reason to kill someone you don't care when you can kill the medic with extra survivability?

[cobalt bay]

The longer you dont kill the more likely you get caught out, if the medic has a head start on you you might need to just kill the random sniper

[gleaming nest]

this weapon gives you more survivability you can just escape and try again to stab the medic instead of wasting your knife on a random player

and if you die with this knife then you would die with stock anyway