This has always been the weakest of any argument against such complaints. As with any concept, it's not about how foreign a concept is to OUR world - it's about how a concept may fit within the world a game has presented.
The Assassin's Creed game is set up to feed players the idea of living out your ancestors past. The game world is set up to convince us that such ancestors would be interacting with key players in history - after all, such imposing and important historical figures met up with all sorts of people, and we don't know half of them. It is not, however, set up to make it convincing that someone can be called an assassin in these worlds and walk around with eighteen tons of weaponry and the most ostentatious outfit around. The game sets up all these elements to suggest we're supposed to be sneaky assassins (hiding in crowds, in bushes, in wells, etc), but simply fails to make the next leap to make the protagonist similarly stealthy in his outward appearance.
But the games have ALWAYS done this. You've always looked hilariously conspicuous wandering around, climbing on top of roofs and parkouring all over the place when almost everyone else is plodding along down below.
The most they ever really did was have you visually "blend in" with that crowd mechanic when you're trying to be incognito, which they do by de-saturating the color, and that's about the extent of it. The AC2 games had various colors for your outfits for people who want to change their basic look, for any reason, and this one is likely to do the same.
But you have never truly blended in. You walk around looking like the most obvious assassin in the world. You're decked out in weapons, with a cowl over your head, running around everywhere like an absurdly physically gifted athlete, and you generally just look menacing and up to no good. And the games have never been too much about stealth, anyway. They have a couple stealth mechanics but you're generally fighting guards out on the open streets with onlookers gaping at you, and the most it generally involves is parkouring out of dodge if it gets too hot and getting involved in rooftop chases.
But typically you don't even have to do that, since you can just murder everyone with ease and then just walk away, and guards reset. They introduced a mechanic in Brotherhood that has you calling your assassin buddies - who greatly resemble you - to leap into the fight in full view of everyone and just wipe fools out. This isn't Thief we're talking about here.