How long should an iPhone 7 last?

I'm not talking about the batteries, because we had them replaced a couple months ago at an Apple store.

Here's the basic situation: I called Sprint yesterday to ask why our cell phone bill has been higher in the past two billing cycles. I assumed it was because a promotional discount hadn't yet kicked in for my oldest son's recent promotional upgrade (we have four lines in total).

Anyway, it turns out that the other three lines we have wrapped up the 18 month contract period and so the three IPhone 7s converted to rental units at $27.00 a phone. We can buy them out at $159 a pop ($477 total) or trade them in on a new 18 month contract for new IPhone 8s at around $10 dollars a month.

So the question we face is: Should we stick without old IPhone 7s or upgrade?

How durable and long lasting is an iPhone?

Added (1). ** "stick without old" should read "stick with our old…"

Years.

With fresh batteries they'll last a good long time. Ignore the sunk cost and reset the clock to consider the offer Sprint presents:
$180 per person over time, gets you new phones that turn into pumpkins after 18 months.
$159 per person up front, gets you 'used' phones you can keep as long as you need to.

Depends on your maintenance of it.

As far as iOS updates about 5/6 years from the time it was introduced.

Phones that are not dropped or dunked in the toilet, washing machine or pools, should last for years. I have several phones that are 20 or more years old, all working perfectly. I witless suggest not buying phones on installments. But if that is the ONLY way you are going to be able to get a phone, I'd at least stop "renting" them.

Depends how you treat them. I have a 6+, my Girlfriend has a 6. I'm not replacing any of them until 5G is in place and then I'll likely wait for a couple of upgrades and not pay $1000 a phone.

I have 6S+ and its about 4 years now. Battery life is still very good also which surprises me. I'll probalby upgrade this year, but yours would certainly go 4 years or more. The thing is even if the battery starts wearing down you can have the apple store put a new one in and it would go much longer even.