I never heard of someone who bought a new phone _solely_ for a new battery
I did.
I had a Samsung Galaxy s10e, and I have an s23 now, which I got on its launch. The only reason I got a new phone is because they don't have legit batteries to replace my shitty s10e battery in my country of residence (very shitty European country). No Samsung support, no iFixIt (they don't ship here), nothing. Just shady shops selling shady batteries.
I preferred the smaller size of the s10e, and software-wise it did everything I needed it to do. Plus I didn't have to rely on Bluetooth earbuds when I need(ed) to wear some earbuds.
Oh, and the battery has gotten noticeably worse in the past year and a half, and the _only_ reason why I will buy a new phone in a year, year and a half is that I know for a fact the battery life will become (even more) unbearable, and there aren't any official/legit channels through which I could replace the battery.
I know I'm talking about Samsung, but the same would apply to Apple here, and any other brand.
If this wasn't an issue for me, I could see myself using the same phone for 5-6 years, probably even longer if set up lineageOS on it -- but after +5 years I imagine I would like to use some new phone with its "new" features... which I will likely never really use in the first place
Every device I've ever wanted to keep after its battery started degrading (mostly laptops, but has happened with a phone too) I've either ordered one from ebay and replaced myself or went to a cheap local shop.
OE batteries aren't magical.
As with buying a battery from ebay, it's still not gonna be an OEM battery (or am I wrong?). OEM stuff is more important for phones than for laptops, I have found
It's a lot more nowadays. Magisk, lsposed, play integrity fork, shamiko, trickystore, custom keybox.xml, zygisk next... and with news of google now forcing certain apps (eg. chatgpt) to be installed from the Play Store + the device being of at least DEVICE integrity it's slowly all falling apart.
To me it seems like throwing money out of the window just for the sake of it.
I think you need to check how much iPhone cost nowadays.
Pro is 1000 USD. That's $16 per month. Easily affordable for most of developing world.
This argument sounds a lot like a used car salesman.
I think focusing on the monthly payment is deceptive. Especially with a device like a phone which could easily meet its doom before the 5.2 years used in your calculation are up.
It's reasonable to compare the total cost of ownership or the up front price. Your choice is a $1000 phone or (say) a $600 phone (new iPhone 14, no tax). There's $400 difference there. That's your money making the choice.
The question of whether it's worth it is something else.
It is for everyone that has is bank account in the red for half of their month. Which is a lot of people regardless if they live in a developing country or not. And those that can save money do not necessarily save money for a smartphone. They do it for their own financial safety. Even the streaming accounts are usually shared accross friends and families and said services don't crack down on them because they know they would simply lose their customers completely anyway. They would rather keep money in case their home need a reparation or their car they are using to go to work break down.
So when the time comes that their phone has issue, many of them are faced with a non scheduled financial issue. They will definitely repair their phone if it is more affordable than buying a new one. No wonder their are lots of repairing phones shops in every city. Where I live you can literally see a queue on the sidewalk during the opening hours. The alternative would be buying a second hand cell phone with the risk of ending up with one whose battery is already in average conditions.
Phones frequently die after 4-5 years.