10 is average, yeah. But your players are supposed to be above average. They're supposed to be someone who's trained with a sword, or who's studied magic for years, or is an expert gymnast, or grew up in the streets learning to steal to survive. Having your players be an inexperienced farmhand who has nothing special about them makes it uninteresting for the players.
Players want to be the person the city calls on when they need help. Even if they aren't "heroes" at level 1, they should still be clearly strong enough to be worth hiring to handle problems. If they're no better at problem solving than Larry, Darrel, and Darrel, then intelligently-written NPCs would have no reason to waste their time on them.
On one hand, I do agree that the players are heroes in contrast to other people. I just don't think that should default them to above average scores. For one thing, the very fact that they have a class is their heroic edge. Even if the PCs start at level 1, the very fact they can eventually learn powerful spells, gain increased sneak attack damage, and more is what sets them apart. Not being born special. Which, actually, is my primary issue with "better by default."
For me, assuming PCs to have a higher ability average is not the best way to approach things thematically. To do so is to say "these people are heroes not because they tried and persevered, but because they were born to." I don't like stories of chosen ones, born heroes, and innate superiority. It's elitist and, in the end, boring. Oh, if course she became a hero, she was always stronger and faster than everyone else.
Ability scores go up so rarely that having 10s across the board means you'll be level 20 by the time you even have a slight advantage in combat based on your scores.
I don't remember suggesting that PCs should have 10s across the board. I said that 10 should be the average, not 12 (or as I've even sometimes seen, frigging 14).
Also, I think you're being a bit hyperbolic there. If we go by 5E, even starting with a 14 isn't bad. You've got a +4 attack bonus, and monster AC hovers around 11-13, which leaves you more likely to hiy than miss. Even high CR monsters don't have ridiculous ACs, and those that do are exceptions. If you're playing a fighter, you can have a 20 Strength by 8th level. Even by the slowest progression, a wizard could have a 20 Int by 12th level, and again they'd ad their proficiency to spell DCs (monsters don't have super-high save bonuses unless they're an exception).
In D&D, particularly modern incarnations, the monsters are literally built to be defeated. Giving players awesome advantages from start is not only unnecessary, I think it pushes things toward boring.