Blue
Ravenous Bugblatter Beast of Traal
That's still not the default assumption of the books. Lv 1 characters are competent. The numbers are scaled with that concept in mind, including the enemies a Lv 1 party is expected to face. If you treat them as brand new commoner, you're just shifting the frame of reference.
I've already said they are heroes. Heroes are competent. I just said that they need not be formally trained.
Okay, let's start again. Here's my position:
First level characters are competent heroes, but at the beginning of their hero game. Some may have experience in things directly relating to their class, such as a sage wizard or a soldier fighter, while others have experience elsewhere, such as the soldier wizard or sage fighter. Their backstory needs to make sense that they have the mechanical capabilities given by their class, be familiarity with armor, ability to cast spells, whatever, but there is no mechanical necessity that they can only pick backgrounds that directly lead into class. I can play the scrappy urchin fighter who learned to fight on the streets as easily as the noble fighter who learned fencing and intrigue at her mother's foot, as easily as the solider fighter, or any other combination that makes a cohesive character. Class descriptions are not prohibitive - they do not bar any ideas that aren't explicitly listed in them.
D&D is a game of high fantasy that can branch out, but at it's core you can survive not only a critical hit from a two handed axe, but also get breathed on by dragons and keep fighting. PCs are heroes, be it by talent, determination, or whatever undescribably factor makes them so. The "beginning of the hero game" is 1st level but still may be more than people with more experience who aren't heroes - e.g. fractional CR NPCs. This isn't to say you couldn't add in level 0 rules for the people who want to start even earlier, but it is entirely possible to play a nobody with talent and/or unconventional training as a first level character. You can be the guild artisan sorcerer who's had a talent for magic and after her caravan was wiped out swore vengance against the brigands that killed her foster-father mentor. You can play veteran of many wars who miraculously survives a battle and makes vow to the gods they they decided to honor and becomes a cleric, even though they weren't trained in a temple (or even have the religion skill).
It's not that it can't be done, but that it's not the default fluff...
The default fluff of classes does not override the fluff and mechanics of backgrounds.