My parents only replaced their iPhone 5 when the battery wasn't holding a charge as well as they'd liked. So, they replaced it with a 5S (this was before the SE, but it was free with a contract anyway). They use it for calls and directions, but not much else so the 5S is more than adequate.
If they still have that 5, it's pretty cheap and quick to do a battery replacement (we charge $49 -- many shops charge similarly.) Should bring it back to life just like new. if nothing else, I think it would be worth doing just to have it as a backup phone.
I have the iPhone 5 now. Until getting this phone, I've never had a cell phone (I don't really get that many phone calls so I haven't bothered). To be honest, the battery life isn't great, but I just bought a small external battery to charge it when necessary. It was cheaper than a replacement and I didn't want to risk making a mistake in the swap. I have more than enough technical skill to do it, but since it's now my only phone I decided not to take the chance. I might do it if the battery life gets too bad.
