With your reasoning Stabbey, assassinate would just be buffed to be closer to the average strength level of the average skill. And thusly it would blend in with many of the already tan blended skills -> Dystopia scenario: "do you want to use one of your three skills in this fight.. they have different colours".
Saying that the only difference between skills is the color of the spell is a strawman argument. Especially considering Larian's history. For DOS 1's Enhanced Edition, Larian rebalanced the game by removing a bunch of skills which were not unique enough (Feather Fall vs Teleport) and establishing each school as having unique access to certain status effects (Only the Scoundrel line had Bleeding, for instance)
Skill A and Skill B can have the exact same base damage, but if Skill A sets Shocked Status and Skill B sets poisoned status, those have two different roles.
Even under that criteria, though, Assassinate is just plain bad. Compare it to another single-target skill, Ballistic Shot:
Assassinate (Huntsman 3, 3 AP, 5 CD) deals 115% damage unstealthed and 165% damage stealthed.
Ballistic Shot (Huntsman 2, 2 AP, 4 CD) deals 100% damage, plus 5% damage per meter between you and the target.
Ballistic Shot equals Assassinate's unstealthed damage at a distance of only 3 meters from the target - practically melee range. When stealthed, Ballistic Shot equals Assassinate's damage at a distance of 13 meters, which is well within bow range even on flat ground.
Ballistic Shot requires 1 less point into Huntsman, is available 7 levels before Assassinate, costs 1 less AP and has 1 turn less cooldown. Nothing is preventing Ballistic Shot from being used 13+ meters away or in Stealth. There is absolutely no niche at all Assassinate fills which Ballistic Shot does not fill equally well or better.
Or take the Air skill "Apportation", which teleports items into the player's inventory. I can't even think of any use for such a useless skill.
My way of thinking is that it is just a bad skill, use it or don't.. Maybe it can be effective in some very specific case if not I still don't care. Basically, asking for a balancing on assassinate is that you just want to nag about something. Like when very very very old ladies complain about the weather
You don't care that Assassinate is bad. Fine. I don't care about your absurd nagging about the idea that "balancing always = bad" or your stupid idea that having bad skills in the game is a good thing.