Sacrificing fun gameplay for realism always is a bad idea.
Is it realistic how the guards react to Altair? Maybe, but it's incredibly annoying when the only acceptable speed is "slow crawl."
Is it realistic that there's only a handful of missions? Maybe, but it becomes repetitive and boring rather quickly.
I mean, if we were being realistic, guards should have no problem picking Ezio out of a crowd in AC2 considering his attire. You'd be constantly getting in fights throughout the game.
Plus, you're saying "it's realistic" to a game where you're reliving a person's life through "genetic memory" in a glorified time machine, and the final fight is use vs. an old man that uses alien technology to clone himself.
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TheRock ~ Slow dramatic zoom-pan. Doesn't phase the hooded man.