Real Advice
I've been in this industry for ten years and I've seen a lot of people throw away perfectly good $1,000 phones just for a new camera and a slightly faster chip. It's a waste of money.
In my experience, you can easily keep a phone for five years if you treat it right. You don't need to be an expert. You just need some basic common sense.
Battery Health (The Big One)
Your battery is the first thing that goes. It's usually dead at 2 years if you charge it to 100% every night. Don't do that.
I think we all need to start using the 80% charge limit. Every flagship has it now. Turn it on. It'll save you $80 on a battery replacement at year three. You'll thank me later.
Storage and Speed
If your phone feels slow, it's usually just full. If you've got 25GB of blurry memes and videos of fireworks that you'll never watch again, delete them. Honestly, cloud backup is your best friend.
I spend about 15 minutes a month just deleting old apps and photos. It keeps the OS happy and your memory fast.
Physical Protection
Get a screen protector. Seriously. I don't care if you're "careful." I've seen a phone drop from 2 feet and shatter. A $10 piece of glass can save you a $300 screen replacement. It's not worth the risk.
In my experience, a thin case is fine. You don't need a bulky Otterbox. Just something to give you a bit of grip.
The Wrap Up
Honestly, just keep your phone. It's fine. It's fast enough. If it's 4 years old and the battery is dying, just pay $100 to get a new battery. It'll feel like a new phone for a fraction of the cost. You don't need the $1,200 "Ultra" just for the shiny new color.