The group I was in when we started our first 3e game started with 3 rogues and a cleric. We snuck up on and sneak attacked just about every encounter we could. We all took 3 levels of rogue before branching out. I went Wizard/Arcane Trickster.
But assuming the fighters do win... well, there's also some monsters that would kill a fighter only party but would be easily defeated by spellcasters. Bat Swarms, for example.
Actually, I've heard it's less just lvl 1, but the lower level in general, up to 5th, although i'd say it stops more at about 3rd. And, as mentioned, if you include splat books and optimization, it becomes different. For example, I once made a Goliath Barbarian 1/Fighter 1 for a game starting at level 3, and slaughtered a lot of people, and dominated the game compared to the caster. I gave him EWP: Fullblade, and Monkey Grip. With 21 Strength, he was doing 4d8+7 with his oversized butter knife.