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My personal opinion is any device may function fine for years to come but I’d always want to be on a supported device. That’s basically any device that is capable of getting the latest iOS update.

So really you’re looking at the SE and 6s and earlier at the moment but anything later is a no go.
 
I replaced my iPhone 6's original battery 3 months ago. It felt like I acquired a new phone. My next iPhone will be the 11 Pro but waiting a bit in hopes of getting a better deal. May buy a refurb when available. I don't feel the need to upgrade every cycle. There's enough in our landfills as it is.
 
80% battery health isn't "used up", it's 80% less powerful than it used to be. The 80% designation that apple gives is their statement that a battery should provide 500 cycles before or when it reaches 80%.

If a big new battery that's twice as big as smaller one (for example 2000mAh vs 1000mAh) degrades over time to 80% capacity, it'll still be a 1600mAh equivalent battery -- still more powerful than a new 1000mAh battery.
 
80% battery health isn't "used up", it's 80% less powerful than it used to be. The 80% designation that apple gives is their statement that a battery should provide 500 cycles before or when it reaches 80%.

If a big new battery that's twice as big as smaller one (for example 2000mAh vs 1000mAh) degrades over time to 80% capacity, it'll still be a 1600mAh equivalent battery -- still more powerful than a new 1000mAh battery.
There's more to that 80% capacity number: https://batteryuniversity.com/learn/article/capacity_loss
 
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