The thing is, there was a minute where to get online, to use a computer, to do these things, you needed basic tech troubleshooting competence.
There was a minute when computing devices came with printed owners manuals that described set-up, usage, and troubleshooting in detail. Many even had a label with a phone number – right on the front of the device – which would connect one to US-based technical support for as many hours as one needed, for a small fee.
Today, if you're lucky, you get an Ikea-style wordless pamphlet that depicts how to plug in the power cord. Then halfway through setting up everything, the device installs an update, reboots, and comes back with a completely different UI. And if you're brave enough to Google for technical support, you end up downloading malware or on the phone with someone in Nigeria who needs you to go buy iTunes gift cards before they can help you.
My WWII-era grandparents had no problems buying a Gateway desktop, setting it up, and spending hours on the phone learning how to do everything that they couldn't otherwise figure out about Window 98 Second Edition or AOL from the novel-length books that came in the box. These days, all you can really do is phone a friend or hope your local library has some free courses.
I generally start looking to replace mine around the time that Google Maps starts becoming laggy. That's usually around the 3 year mark for me. After 4 years things get pretty bad.
Nexus 5 -> Pixel 2 -> Pixel 6a
Practically every app update grows its respective compute and memory footprint. And over time, it adds up. Combine that with the big jumps in resource usage that come with OS updates, and eventually things just start slowing down.