green slime said:
But... If this mythical "sweet spot" is merely mathematically driven (such as 50% or 70%), and constant with level-appropriate challenges, what is really the point of levelling?
The monsters you fight change. A lot.
At 1st level, you had a 70% chance to hit an Orc, and probably survive your encounter against it (assuming you have few comrades)
At 4th level, you have a 70% chance to hit an Ogre, a monster you had little chance of beating earlier. And the Orc becomes more of a nuisance - groups of Orcs, wow that's different (at least ideally

)
At 10th level, you fight a Dragon 8 times larger than you. The Orcs? Laughable? The Ogre? Maybe in groups...
That changes a lot in feeling. And that is what it is all about, because in the end, you still expect to somehow prevail (most the time), because you're not here to play "Krusk, the heroic Barbarian who was killed after a dozen battles against Orcs." You're more interested in "Krusk, the legendary Barbarian who slaughtered the Horde of Goblins that wanted to invade Littletown, slayed the Dragon Vallisus on top of the Huge Spire of Despair, and died valiantly in the battle of Slaugtherhouse, while destroying the Avatar of the Demon Prince of Snakes..."*)
*
Except, off course, for the cheesy names