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The real reason 50 became "normal" was because Leica originally promoted it as such, and set the precedent. If they'd chosen 35, 40, or 58 that's what everybody else would have used too.
However with OP's intended usage of "architecture street photography" I'd say get the widest lens you can. In the street ability to "zoom with your feet" is often limited. There might be a wall or something limiting how far back you can get. It's usually easier to get closer to things than to get further from them.
