Hate to retire it if all I need to do is subscribe to malware prevention...
You definitely don't need to retire it. Until quite recently a small laptop of mine was running Win8.1. The only reason it doesn't still is that an update to Win8.1 broke the dual-boot into Linux and, as I no longer have to test sowtware compatibilty with older Windows versions, I just blew away Win8.1 and installed a new Linux on the whole drive.
I still have a Win2K laptop here for an old probram that won't run on later Wins. It was working fine last time I checked, but the battery is now really strugling and the display hinges are cracked.
If you have a broadband router, it probably has a firewall, so another option is to use that to block access to/from the Internet.
I think Microsoft is trying to frighten/bully people into getting more recent versions and that's becoming a significant issue now they're tieing OS versions to CPU versions.
A little bit of Linux advocacy. All but one of my PCs run Linux, only a few have Windows. For most everyday stull Linux is excellent, though one typically uses platform agnostic applications, rather than Windows or Mac targetted applications. Firefox/Chrom*/Opera/etc., Thunderbird, LibreOffice, etc., all of which are mature and stable. It doesn't work out so well with BiaB, but a number of DAWs are available (Ardour, Reaper, BitWig, Mixbus,). Liunux doesn't require us to download drivers and
most hardware 'just works'.
If/when a PC ever seems to no longer allow Windows, consider the switch. There are a number of very user-friendy Linux distributions out there ... Mint, Arch, Ubuntu,...).
An against: Older machines do typically use more power per performance than newer machines. Doing the power-cost calculations makes keeping old machines look less attractive.