occam
Hero
Assassinate is now bad, really bad.
at 3rd level you get +3 damage on one attack, in 1st round of combat, only on targets with worse initiative.
Extra sneak attack damage from a crit is about 1.75 points/level. Is 1 point/level less? Yes, BUT... this assassin doesn't need to achieve surprise on the opponent.
Taking your level 3 example, the assassin will score an extra 3 points of damage instead of an (average) 7 points. Giving up 4 points of damage doesn't seem like much when you can do it at the start of every combat, instead of just those encounters where conditions are amenable to gaining surprise. Plus, you could roll an actual crit, and still get the Assassinate bonus damage.