they will never do it
It would be interesting if there were feats or enhancements, or even gear that enabled this. For example, what if full stacks of MtF enabled a 10% cooldown reduction, or each failed assassinate increased the DCs and facilitated the cooldown (the former like QP)?
The key thing is that assassinate keeps you hidden unlike other instakills. I don’t mind if I am in a group and others are popping heads constantly before I get to them.
As long as my toon is alive and looting at the end, all good…