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Pretty much exclusively? The last 4-5 iPhone purchases in my family have all been due to dying batteries (plus a couple of off-brand battery replacements by local cellphone techs).
Nothing else on iPhones really ever breaks, provided you keep some sort of case on it. The only non-battery failure I've ever had was a corroded lightning port (on a iPhone that was regularly used in a salt-chlorine swimming pool). And of course a couple of replacements due to critical banking apps that have drop support for old iOS versions...