The question is wrong
It should be "Should The Government be legally required to extend support for systems still in use in front line public services?" closely followed by "Should software suppliers to front line public services be required to update their software to be compatible with OSes <10 years old?"
Because some services will need to keep running software that simply doesn't run on Windows 7 or above with no upgrade path to one that does. That's how you get XP that won't die.
Also: For a lot of cash strapped public services, dosh earmarked for IT Infrastructure upgrades/improvements can quickly find itself being diverted into the budget for directly supplying those services. Ringfencing that cash with a legal responsibility to meet a minimum standard for IT might help concentrate minds in the right area.