I think this is a function of the lens you view the game through, rather than a necessary extension of what would happen if all players were from the same class. Certainly, a party full of assassins is more likely to engage in stealth than a party full of fighters, but it doesn't preclude combat from being the focus. Remember that game Assassin's Creed where melee was a great idea?
Fighters play the combat system and are designed to be adept at it. A 0-level violin maker can try fighting too, but he is very likely to fail miserably and won't have much of a future in it (or any future) unless he trains to become a fighter.
On the flipside, Fighters suck at making violins. Does this mean they can't try and make one? Of course not, but their is no game system to support that game in D&D.
We have a combat system for fighters, a magic system for magic-users, and a clerical system for clerics. Thieves sort of try and do what everyone does (move, hide, search, etc.), but aren't good at any of them. At best they are good at avoiding all three of the game systems to get ahead via material ends alone.
In the end, assassins are fighter/thief mixes anyways, so Assassin's Creed makes sense if they didn't design like "Thief" (I haven't played the former).
I still say the game radically differs for play because of each class's design and the rules in play for each. If someone were to make a violin crafting game system for a 4th scope of play, then perhaps a single class party of violin makers could focus on such most of the game. (Not that combat, magic, or clericism wouldn't peripherally occur - especially if the game supports those other options already).
