If it runs fine with its current OS and hardware, there's no need to upgrade. Keep your current OS, your current hardware and you're fine. It's not Microsofts duty to support very old hardware.
Actually it is, Windows 10 is supported until 2020 and I think it's entirely reasonable to expect all Win10 hardware to be updateable until then as MS very clearly say the OS is not supported without all updates installed. Cf support.microsoft.com/en-gb/help/13853/windows-lifecycle-fact-sheet
That's not an answer to the question of what Windows 10 can do that makes my hardware obsolete? It's definitely not performance.