6D6 is only 21 damage and it's situational - you have to expose the target to Sneak Attacks, and they must not be immune to precision damage
Spending a mythic feat on it is basically not relevant
Anyway I never said Sneak dice are bad, I'm just saying base damage scales better
Which it clearly does
6d6 but is akshualy 8d6, (10d6 if deliverer) if you factor in 1 feat and another mythic then if everything is relevant.
But still 21 dmg is already superior?
As a slayer you get a 1d6 sneak dice every 3 level.
Now compare with what you call "base dmg" and you say scales better.
+2 dmg after level 5.
Another +2 at level 12.
And roughly every 4 levels starting at level 5 you get +1 dmg through weapon training.
And assuming you get a mythic weapon specialization, you will have +3 dmg at the start of chapter V, so basically for most of the game, to finally reach +5 for the last 20 mins.
Fighter:
+4 weapon specialization feats
+5 mythic specialization
+4 weapon training
Slayer:
+5 studied target
+28 sneak attacks (8d6)
Obviously with a crit you get more damage from a fighter but overall sneak damage is more consistent, and scales better since it increases every few levels by itself, so you will do more damage for most of the game with a slayer, which is my main point.
Beside usually the enemies that are immune to sneak attacks are also immune to criticals, so your point that precision doesn't get multiplied is moot, and in the game is so easy to make enemies vulnerable to precision damage that it's harder to avoid it.
Sneak damage also isn't guaranteed. Lots of enemies with outright immunity, or true seeing and you aren't riding a pet to threaten them on your first turn when you're making your initial attack. The more optimized you are the more likely you're taking down enemies in 1 or 2 rounds so you might be sneak attacking only half the time.
Also, don't forget your mutagens. Str bonuses also apply both to AB and to damage. Obviously it's not easy to generically calculate how much +1 AB adds compared to +1 damage, but I'd argue it's at least equivalent (outside of meme screenshots where you self-select the round where you roll all 20s).
Enemies immune to sneak are usually immune to crits, but yeah as I said for most of the cases sneak is better, not all the time, hence slayer does more damage usually.
MW get more AB, I don't discuss it (especially because quarry is sadly insight so shouldn't stack with a lot of things), just talking about DPS.