You raise a good point. My iPhone is about 3 1/2 years old. I cracked the screen about a year or so after I got it. Although it was annoying I cracked the screen, it is barely noticeable during use. About six months ago or so, I had to start fiddling with the charging cable to get the phone to charge. If I was traveling more or had to rely on the phone to run a business, I would have a new phone.
I haven't used a screen protector in over a decade. Those make the screen look much worse. So I just use my phone "naked" as those screens are pretty durable. I do surround my phone with a rubber bumper so if I drop it (as I often do), it's no big deal.
I only replace it when the battery life fails to get me through an entire day without recharging. Two and a half years in, I'm still getting through the day with the one I have. Plug it in at night and I'm good for another day.