What Your Phone Has Replaced
Modern smartphones might be some of the most useful devices ever invented. I can't think of anything else that has completely invalidated so many other things just by existing. I remember when I was a kid, we had lots of dedicated devices for different tasks. You had a camera for taking pictures, a GPS for getting around, a CD player or MP3 player for listening to music. But my phone has completely replaced all of those.
Camera
Back in the day, if you wanted to take pictures or video, you needed a camera. We even had those disposable cameras where you needed to take it to a store to get it developed (crazy that we used to just trust random strangers with all our personal photos). Nowadays your phone's camera lets you take photos and videos without having to lug around a whole dedicated camera or camcorder. Of course, professionals will still want to use a dedicated camera, but for the average person, there's really no reason to bother with one anymore.
GPS
When we got our first TomTom GPS (don't hear much about them anymore), I remember how much easier it made family trips. No more fumbling with a map, trying to figure out where you are, or printing off directions from the internet to try and follow. Now there's no need for a dedicated GPS anymore. The same way the GPS invalidated maps, smartphones invalidated GPS's.
Landline
It used to be almost impossible to imagine not having a landline in your house. Even though cell phones existed, a landline was much more reliable. Nowadays with our much improved cellular networks, most people have a reliable phone in their pocket at all times. The concept of having a household phone that you all share seems to have given way to everyone having their own individual phone number. I don't miss it, the loud ringing that would blare out was very annoying and I didn't like having potentially private calls ringing out for everyone in the house to hear.
Step Counter
Our gym coach always had us attach step counters to our waist when we went for runs in school. These were common enough to see back then, people wanted to track how many steps they got in each day. But they were pretty ugly and it was annoying having a little plastic thing clipped to your shirt or pants. Conveniently, smartphones have built-in acceleromters and gyroscopes and they tend to sit around your waist in your pocket, so they can count your steps for you. Even more, they can record your step counts over time to show you have much activity you're getting, and they can alert you to problems like instability in your walk cycle. There is absolutely no reason to buy a dedicated step counter anymore.
Timer
It used to be common to have a physical timer to time how long you're cooking things for. You might also have had a stopwatch for timing your runs maybe. Now it seems quaint to have a comletely dedicated timer for just about anything.
Things Your Phone Will Replace
There's a few things that I think phones have the potential to dethrone but they're just not quite there yet.
Thermometer
The Google Pixel 8 Pro introduced a thermometer to the phone to measure the temperature of objects. It's a neat idea, and if it became widely available then there would again be no need for such a device in your home. You could also imagine an ambient temperature sensor that would let you know what the temperature of your environment is.
Game Console
If you want to game without any hassle, dedicated game consoles have always been the go-to. They provide the power needed to play the latest games at a good price. Portable consoles like the Nintendo Switch and the Steam Deck have been massively popular for their portability as well. But being able to play AAA games on your phone would be even more portable, since you're going to have your phone with you all the time anyway. Now we're starting to see AAA games like the Resident Evil 4 remake showing up on Apple's app store. There's also been increased support for controllers on mobile. It's only a matter of time before these dedicated portable consoles are a thing of the past.
Home PC
Imagine being able to come home, plug your phone into your monitor to charge, and use your phone as your PC. It's not as far-fetched as it might sound, with Android's upcoming desktop mode. Most things you used to need a PC for are now possible on a phone, with the major exception of programming. However I think most people don't really want to deal with owning a whole PC when their phone will do everything they need.
Measuring Tape
There's already apps that use your phone's camera to judge the distance between two points in space, but I'm putting this here because it's not quite accurate enough yet. With some more refinement I don't think we'll need to carry around measuring tape anymore.
Home Internet
With 5G, it's now possible to get speeds in excess of 1 Gb/s, more than enough for most people's needs. I personaly use T-Mobile home internet already, and it's been great. But I would really love to be able to use my phone as my router in my home, and just rely on my 5G phone plan instead of having to pay for my phone plan *and* a dedicated home internet service. The only thing stopping me right now is the restrictions carriers put on mobile hotspot data. Most of them give you some amount of high-speed data with your plan but restrict you to 2G speeds when you run out. I know there have been issues with 5G's rollout, but as it matures and 6G looms ever closer I expect these hotspot restrictions to ease up.