Computer slowing down: Windows 10 has different refresh functions to overcome that. One keeps apps and user data, another one keeps data but (re-install on its own?) apps. And of course the classic Windows reinstall, but few consumers have time for that.
Note that Windows 8.1 will go end of Support at January 2023, when even security updates end.
Windows 10 will continue without a support end announced. Just if you don't accept the function updates anymore, each one version of Windows 10 will go end of Support 18 months after release.