Couple of things:
First, there are so many myths about the American West, what it was like, what people did, that it's hard separating fact from fiction. It's not at all sure that in any real western town the residents would have lined up behind a sheriff in a fight with a band of outlaws. In real life there were never any guarantees of what would happen; each situation and each town was different. Many townspeople did believe law and order was "what they paid a sheriff for" and would not have gotten into the middle of a fight. The point is, such things could go either way.
Second, in this film remember that not everybody in the town was on Kane's side. On the contrary, lots of people disliked him and liked Miller. The saloon, the hotel were full of such people, and presumably there were more. Sure, there were a lot of "good people" who supported what Kane had done but even if men had stepped forward to help him it's not as if he'd have had the whole town with him in a mass showdown. Many of Miller's friends might have taken sides with Miller and his three henchmen in such an all-out fight.
Third, a lot of people were cowed into not fighting by the argument that big businesses were looking to move into the town and making it prosperous, and that if they heard about gun play in the streets that would deter them from coming. Of course, nobody says anything about businesses' attitudes to a town that doesn't stop a band of killers from riding in, scaring a sheriff off and not having one person stand up for law and order; I don't think that would be conducive to investment either. But that was one argument used to scare people into inaction.
But as to the OP's point, I agree: alone or not, but especially alone, if I were Kane I'd have gone to a rooftop and picked at least one or two of Miller's men off as they came in. The character of Kane as written makes him seem more concerned with the appearance of needing to be fair and honorable than in simply winning a life-or-death fight against lawbreakers.
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