For example they are not provoding security upgrades. Now you can unlock your bootloader and install your own Android Rom built by yourself or fron someone else. However with Safetynet, many applications are not working if you jave unlocked your bootloader. So, you can run the latest version but without lockec bootloader, you cannot use for example some banks. They enforce you to have Google Play Services controlling your phone to have access to your money. So they can pretty easily just sop providing updates and then you sre enforced to buy a new one. The last years Samsung and Pixels are getting more than 5years of support however nowadays our devices are powerful enoughto be used for almost decade.