
I highly doubt that. It's possible but unlikely as batteries have a lifecycle of around 2000+ charges before that battery starts to lose it's charge at a faster rate and even then we're talking about something miniscule by about 2-3%.
The average person will charge their phone once a day so that theoretically is about 365 charges (almost) therefore if charged everyday since September 2015 then that's around 2/5 of it's lifecycle used already and the battery's performance shouldn't dip before then and even if it did it would be unnoticeable.
My iPhone 4S works completely fine however what's slowing the phone down is the bloatware iOS which wasn't designed for the 4S and has way too many kernel processes going on in the background that the CPU can handle.
The only thing that would force me to upgrade is if an app needs a mandatory firmware upgrade in order for it to work efficiently especially in an app that I need like banking, Whatsapp, Instagram, Chrome, Google Drive is all I use and those companies like money so I doubt they'll leave me in the dust

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Also iPhone charger's don't have fast charging capabilities and are capped at around 1.6A even on a 2A charger, but the charger supplied is 1A and this is a very safe output rating that isn't even close to damaging a battery. Therefore, it's highly unlikely for the battery to become degraded as much as you think. This problem would be more prevalent with Android devices and iPhone tablets as they require or are capable of charging at 2A. Though it would charge it faster it at the same time will definitely degrade the battery twice as fast.