ELI5 Skyscrapers - which NYC skyscraper could have fallen over?

Are the 100+ year old skyscrapers still safe? How do skyscrapers get built straight up without any leaning or imperfections or off angle walls? How does the concrete get poured so perfectly? How have designs changed over the years? What NYC had a 6% chance of falling in any given year?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: troyisawinner, justasking1t, johnproof, monkshabit, torostein, hopefulmonstr, hippityhoppityboop, flannelheart and krisalyssa

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